make "child-clean-exit" the default option

Fix #408. The destructor of NW Shell will try to shutdown and kill
the renderer immediately:

content::RenderProcessHostImpl::Cleanup (this=0xf683b40) at ../../content/browser/renderer_host/render_process_host_impl.cc:1159
0x0a16e614 in content::RenderProcessHostImpl::Release (this=0xf683b40, routing_id=1)
at ../../content/browser/renderer_host/render_process_host_impl.cc:1140
0x0a194d93 in content::RenderWidgetHostImpl::~RenderWidgetHostImpl (this=0xf6a2884, __vtt_parm=0xf4a5dfc, __in_chrg=<optimized out>)
at ../../content/browser/renderer_host/render_widget_host_impl.cc:203
0x0a17ac9c in content::RenderViewHostImpl::~RenderViewHostImpl (this=0xf6a2880, __in_chrg=<optimized out>, __vtt_parm=<optimized out>)
at ../../content/browser/renderer_host/render_view_host_impl.cc:199
0x0a17ad21 in content::RenderViewHostImpl::~RenderViewHostImpl (this=0xf6a2880, __in_chrg=<optimized out>, __vtt_parm=<optimized out>)
at ../../content/browser/renderer_host/render_view_host_impl.cc:215
0x0a19ac9f in content::RenderWidgetHostImpl::Destroy (this=0xf6a2884)
at ../../content/browser/renderer_host/render_widget_host_impl.cc:1375
0x0a1957bd in content::RenderWidgetHostImpl::Shutdown (this=0xf6a2884)
at ../../content/browser/renderer_host/render_widget_host_impl.cc:311
0x0a18045a in content::RenderViewHostImpl::Shutdown (this=0xf6a2880) at ../../content/browser/renderer_host/render_view_host_impl.cc:1083
0x0a39cde9 in content::RenderViewHostManager::~RenderViewHostManager (this=0xf6a1bd8, __in_chrg=<optimized out>)
at ../../content/browser/web_contents/render_view_host_manager.cc:54
0x0a1d8ab2 in content::WebContentsImpl::~WebContentsImpl (this=0xf6a1a80, __in_chrg=<optimized out>)
at ../../content/browser/web_contents/web_contents_impl.cc:328
0x0a1d8bbb in content::WebContentsImpl::~WebContentsImpl (this=0xf6a1a80, __in_chrg=<optimized out>)
at ../../content/browser/web_contents/web_contents_impl.cc:378
0x08542f36 in base::DefaultDeleter<content::WebContents>::operator() (this=0xf813a50, ptr=0xf6a1a80) at ../../base/memory/scoped_ptr.h:138
0x085426c3 in base::internal::scoped_ptr_impl<content::WebContents, base::DefaultDeleter<content::WebContents> >::~scoped_ptr_impl (this=
0xf813a50, __in_chrg=<optimized out>) at ../../base/memory/scoped_ptr.h:221
0x085424dd in scoped_ptr<content::WebContents, base::DefaultDeleter<content::WebContents> >::~scoped_ptr (this=0xf813a50,
__in_chrg=<optimized out>) at ../../base/memory/scoped_ptr.h:305
0x08540537 in content::Shell::~Shell (this=0xf813a20, __in_chrg=<optimized out>) at ../../content/nw/src/nw_shell.cc:126
0x085405c7 in content::Shell::~Shell (this=0xf813a20, __in_chrg=<optimized out>) at ../../content/nw/src/nw_shell.cc:142