Fix a hang on quit when application cache is used

If you navigated to a page that used the HTML Application Cache, you'd see a hang on quit with a backtrace like so:

```
+     2825 content::ContentMain(int, char const**, content::ContentMainDelegate*)  (in libchromiumcontent.dylib) + 64  [0xb33190]
+       2825 ???  (in libchromiumcontent.dylib)  load address 0x1c9000 + 0x96b261  [0xb34261]
+         2825 ???  (in libchromiumcontent.dylib)  load address 0x1c9000 + 0x96a409  [0xb33409]
+           2825 content::BrowserMain(content::MainFunctionParams const&)  (in libchromiumcontent.dylib) + 200  [0x14290b8]
+             2825 ???  (in libchromiumcontent.dylib)  load address 0x1c9000 + 0x1265426  [0x142e426]
+               2825 content::BrowserMainLoop::~BrowserMainLoop()  (in libchromiumcontent.dylib) + 17  [0x14294a1]
+                 2825 content::BrowserMainLoop::~BrowserMainLoop()  (in libchromiumcontent.dylib) + 357  [0x1429625]
+                     2825 brightray::BrowserMainParts::~BrowserMainParts()  (in ) + 70  [0x68df6]
+                       2825 scoped_ptr<brightray::BrowserContext, base::DefaultDeleter<brightray::BrowserContext> >::~scoped_ptr()  (in ) + 23  [0x68ff7]
+                         2825 scoped_ptr<brightray::BrowserContext, base::DefaultDeleter<brightray::BrowserContext> >::~scoped_ptr()  (in ) + 23  [0x69297]
+                           2825 base::internal::scoped_ptr_impl<brightray::BrowserContext, base::DefaultDeleter<brightray::BrowserContext> >::~scoped_ptr_impl()  (in ) + 23  [0x692b7]
+                             2825 base::internal::scoped_ptr_impl<brightray::BrowserContext, base::DefaultDeleter<brightray::BrowserContext> >::~scoped_ptr_impl()  (in ) + 50  [0x692f2]
+                               2825 base::DefaultDeleter<brightray::BrowserContext>::operator()(brightray::BrowserContext*) const  (in ) + 46  [0x6916e]
+                                   2825 brightray::BrowserContext::~BrowserContext()  (in ) + 127  [0x672bf]
+                                     2825 base::SupportsUserData::~SupportsUserData()  (in libchromiumcontent.dylib) + 57  [0xccc019]
+                                       2825 ???  (in libchromiumcontent.dylib)  load address 0x1c9000 + 0xb03230  [0xccc230]
+                                         2825 ???  (in libchromiumcontent.dylib)  load address 0x1c9000 + 0xb0324e  [0xccc24e]
+                                           2825 ???  (in libchromiumcontent.dylib)  load address 0x1c9000 + 0x14c4239  [0x168d239]
+                                             2825 content::StoragePartitionImpl::~StoragePartitionImpl()  (in libchromiumcontent.dylib) + 17  [0x16899a1]
+                                               2825 content::StoragePartitionImpl::~StoragePartitionImpl()  (in libchromiumcontent.dylib) + 491  [0x1689bab]
+                                                 2825 content::ChromeAppCacheService::DeleteOnCorrectThread() const  (in libchromiumcontent.dylib) + 66  [0x1424f32]
+                                                   2825 content::ChromeAppCacheService::~ChromeAppCacheService()  (in libchromiumcontent.dylib) + 50  [0x1424e32]
+                                                     2825 appcache::AppCacheService::~AppCacheService()  (in libchromiumcontent.dylib) + 301  [0x2b8ad2d]
+                                                       2825 ???  (in libchromiumcontent.dylib)  load address 0x1c9000 + 0x29c9b81  [0x2b92b81]
+                                                         2825 ???  (in libchromiumcontent.dylib)  load address 0x1c9000 + 0x29c9da4  [0x2b92da4]
+                                                           2825 appcache::AppCacheDiskCache::~AppCacheDiskCache()  (in libchromiumcontent.dylib) + 17  [0x2b78c71]
+                                                             2825 appcache::AppCacheDiskCache::~AppCacheDiskCache()  (in libchromiumcontent.dylib) + 135  [0x2b78d17]
+                                                               2825 disk_cache::BackendImpl::~BackendImpl()  (in libchromiumcontent.dylib) + 17  [0x25979a1]
+                                                                 2825 disk_cache::BackendImpl::~BackendImpl()  (in libchromiumcontent.dylib) + 305  [0x2597af1]
+                                                                   2825 base::WaitableEvent::Wait()  (in libchromiumcontent.dylib) + 50  [0xccd942]
+                                                                     2825 base::WaitableEvent::TimedWait(base::TimeDelta const&)  (in libchromiumcontent.dylib) + 347  [0xccdb3b]
+                                                                       2825 base::ConditionVariable::Wait()  (in libchromiumcontent.dylib) + 35  [0xcccbb3]
+                                                                         2825 pthread_cond_wait$UNIX2003  (in libsystem_c.dylib) + 71  [0x964d3089]
+                                                                           2825 _pthread_cond_wait  (in libsystem_c.dylib) + 833  [0x9644d280]
+                                                                             2825 __psynch_cvwait  (in libsystem_kernel.dylib) + 10  [0x94b8e8e2]
```

`BackendImpl` was waiting on `BrowserThread::CACHE`, but that thread had already been stopped. The solution is to destroy the BrowserContext before threads have been stopped. We now do this in `BrowserMainParts::PostMainMessageLoopRun`, which matches content_shell.