Revert of compositor-worker: Share a thread and an isolate for compositor workers.

Revert of compositor-worker: Share a thread and an isolate for compositor workers. (patchset #13 id:300001 of https://codereview.chromium.org/1158443008/)

Reason for revert:
Fails to compile on "WebKit Win x64 Builder",

 http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Win%20x64%20Builder/builds/49353

Original issue's description:
> compositor-worker: Share a thread and an isolate for compositor workers.
> 
> Notable changes:
> 
> . WorkerThread: Move thread initialization/shutdown into virtual methods, so
>   that CompositorWorkerThread can override the behaviour to allow multiple
>   compositor workers to safely share the same thread.
> . CompositorWorkerManager: Responsible for managing the WebThreadSupportingGC
>   and v8::Isolate for the compositor workers. It essentially keeps a ref-count
>   for the thread and isolate and decides when to shutdown/initialize a thread
>   or isolate.
> 
> BUG=436932
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196625

TBR=kinuko@chromium.org,haraken@chromium.org,sadrul@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=436932
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196626

[sof, 2015-06-06 08:55:43]

Created Revert of compositor-worker: Share a thread and an isolate for
compositor workers.

[commit-bot: I haz the power, 2015-06-06 08:55:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1166923003/1

[commit-bot: I haz the power, 2015-06-06 08:56:21]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196626

[haraken, 2015-06-06 10:29:20]

LGTM