Don't leak Documents when using MutationObserver from extensions

Don't leak Documents when using MutationObserver from extensions
https://bugs.webkit.org/show_bug.cgi?id=111234

Patch by Elliott Sprehn <esprehn@gmail.com> on 2013-03-01
Reviewed by Adam Barth.

.:

* ManualTests/leak-observer-nonmain-world.html: Added.

Source/WebCore:

MutationObserverCallback holds a WorldContextHandle which secretly isn't
a handle to anything when it's for the main world. When it's for a non-main
world though, like those used in extensions, it becomes a strong reference
to the v8::Context which results in leaks by creating cycles:

MutationObserver -> Callback -> World -> Document -> Node -> MutationObserver.

Instead we should keep a RefPtr to a DOMWrapperWorld in the callback and then
get the v8::Context from that inside handleEvent.

Tests: ManualTests/leak-observer-nonmain-world.html

* bindings/v8/V8Binding.cpp:
(WebCore::toV8Context): Added overload that takes a DOMWrapperWorld.
* bindings/v8/V8Binding.h:
* bindings/v8/V8MutationCallback.cpp:
(WebCore::V8MutationCallback::V8MutationCallback):
(WebCore::V8MutationCallback::handleEvent):
* bindings/v8/V8MutationCallback.h:
(V8MutationCallback):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@144522 268f45cc-cd09-0410-ab3c-d52691b4dbfc