Implement Context::GetIsolate()

Implement Context::GetIsolate()

BUG=
TEST=test-api.cc (IsolateOfContext)

Committed: http://code.google.com/p/v8/source/detail?r=13397

[haraken, 2013-01-16 09:44:41]

WebCore calls V8 bindings here and there. At entry points from WebCore to V8
bindings, we have a Frame or a ScriptExecutionContext, which holds an
Persistent<Context> associated with it. So we need a way to retrieve an Isolate
from the Persistent<Context> to pass the Isolate around in V8 bindings.
(Currently we have to call Isolate::GetCurrent() to get an Isolate.)

Could you review it?

[Michael Starzinger, 2013-01-16 10:14:15]

Sven, I think you should review this, since you are currently working on
Isolates in our API.

[Sven Panne, 2013-01-16 12:05:40]

LGTM. As discussed offline, in the long run one might have to reconsider the
alternative of storing the Isolate explicitly in the Chrome bindings, e.g.
ScriptExecutionContext and friends.

I'll land this CL for you...