[V8] ScheduledAction::m_context can be empty, so we shouldn't
retrieve an Isolate by using m_context->GetIsolate()
https://bugs.webkit.org/show_bug.cgi?id=109523

Reviewed by Adam Barth.

Chromium bug: https://code.google.com/p/chromium/issues/detail?id=175307#makechanges

Currently ScheduledAction is retrieving an Isolate by using m_context->GetIsolate().
This can crash because ScheduledAction::m_context can be empty. Specifically,
ScheduledAction::m_context is set to ScriptController::currentWorldContext(),
which can return an empty handle when a frame does not exist. In addition,
'if(context.IsEmpty())' in ScheduledAction.cpp implies that it can be empty.

Alternately, we should pass an Isolate explicitly when a ScheduledAction is instantiated.

No tests. The Chromium crash report doesn't provide enough information
to reproduce the bug.

* bindings/v8/ScheduledAction.cpp:
(WebCore::ScheduledAction::ScheduledAction):
(WebCore):
(WebCore::ScheduledAction::~ScheduledAction):
* bindings/v8/ScheduledAction.h:
(ScheduledAction):
* bindings/v8/custom/V8DOMWindowCustom.cpp:
(WebCore::WindowSetTimeoutImpl):
* bindings/v8/custom/V8WorkerContextCustom.cpp:
(WebCore::SetTimeoutOrInterval):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142565 268f45cc-cd09-0410-ab3c-d52691b4dbfc