Revert of Make DOMWrapperWorld::current() return a reference instead of a pointer (https://codereview.chromium.org/209713003/)

Also reverting dependent patch https://codereview.chromium.org/213543004

Reason for revert:
Likely to have caused memory leaks:
http://build.chromium.org/p/chromium.memory/builders/Linux%20ASan%2BLSan%20Tests%20%282%29/builds/989/steps/browser_tests/logs/stdio#failure1

(1)
Direct leak of 32 byte(s) in 1 object(s) allocated from:
    #0 0x4b50d1 in operator new(unsigned long) /usr/local/google/home/hwennborg/chromium/src/third_party/llvm/projects/compiler-rt/lib/asan/asan_new_delete.cc:62
    #1 0x9bdfd8a in WebCore::V8PerContextDataHolder::install(v8::Handle<v8::Context>, WTF::PassRefPtr<WebCore::DOMWrapperWorld>) third_party/WebKit/Source/bindings/v8/V8PerContextData.cpp:56
    #2 0x9bdfa06 in WebCore::V8PerContextData::V8PerContextData(v8::Handle<v8::Context>, WTF::PassRefPtr<WebCore::DOMWrapperWorld>) third_party/WebKit/Source/bindings/v8/V8PerContextData.cpp:108
    #3 0x9bc1afd in WebCore::V8PerContextData::create(v8::Handle<v8::Context>, WTF::PassRefPtr<WebCore::DOMWrapperWorld>) third_party/WebKit/Source/bindings/v8/V8PerContextData.h:66:16
    #4 0x9be64fc in WebCore::V8PerIsolateData::ensureDomInJSContext() third_party/WebKit/Source/bindings/v8/V8PerIsolateData.cpp:132

(2)
Indirect leak of 40 byte(s) in 1 object(s) allocated from:
    #0 0x4b50d1 in operator new(unsigned long) /usr/local/google/home/hwennborg/chromium/src/third_party/llvm/projects/compiler-rt/lib/asan/asan_new_delete.cc:62
    #1 0x9b2ee18 in operator new third_party/WebKit/Source/bindings/v8/DOMWrapperWorld.cpp:59:22
    #2 0x9b2ee18 in WebCore::DOMWrapperWorld::create(int, int) third_party/WebKit/Source/bindings/v8/DOMWrapperWorld.cpp:53
    #3 0x9be64e7 in WebCore::V8PerIsolateData::ensureDomInJSContext() third_party/WebKit/Source/bindings/v8/V8PerIsolateData.cpp:132
 
(3)
Indirect leak of 24 byte(s) in 1 object(s) allocated from:
    #0 0x4b46c1 in __interceptor_malloc /usr/local/google/home/hwennborg/chromium/src/third_party/llvm/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:75
    #1 0x62caf0a in partitionAllocGenericFlags third_party/WebKit/Source/wtf/PartitionAlloc.h:533
    #2 0x62caf0a in partitionAllocGeneric third_party/WebKit/Source/wtf/PartitionAlloc.h:549
    #3 0x62caf0a in WTF::fastMalloc(unsigned long) third_party/WebKit/Source/wtf/FastMalloc.cpp:125
    #4 0x9b2ed61 in operator new third_party/WebKit/Source/wtf/RefCounted.h:175
    #5 0x9b2ed61 in WebCore::DOMWrapperWorld::create(int, int) third_party/WebKit/Source/bindings/v8/DOMWrapperWorld.cpp:53

Original issue's description:
> Make DOMWrapperWorld::current() return a reference instead of a pointer
> 
> Now that it's guaranteed that DOMWrapperWorld is not 0 at any given time, we can make it a reference instead of a pointer.
> 
> BUG=341032
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170261

TBR=jochen@chromium.org,dcarney@chromium.org,abarth@chromium.org,haraken@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=341032

Review URL: https://codereview.chromium.org/217053007

git-svn-id: svn://svn.chromium.org/blink/trunk@170357 bbb929c8-8fbe-4397-9dbb-9b2b20218538