Introduce ExecutionContextClient

Introduce ExecutionContextClient

This extracts most of the virtual functions from ScriptExecutionContext
to newly introduced ExecutionContextClient interface, which Document
and WorkerGlobalScope implement.

ScriptExecutionContext::suspendActiveDOMObjects(),
resumeActiveDOMObjects(), stopActiveDOMObjects() are devirtualized
as there are no overrides of them.

This is a preparation for decoupling ScriptExecutionContext
from Document inheritance chain.

This is re-landing r159421 which was reverted at r159599 due to reported crashes.
This change now have some NULL checks to prevent such crashes.

BUG=305497, 306880
TEST=none
R=haraken@chromium.org
TBR=abarth@chromium.org, dglazkov@chromium.org, tkent@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=159656

[Hajime Morrita, 2013-10-15 08:56:01]

Rubberstamp please? This is relanding a reverted CL.

[haraken, 2013-10-15 08:59:09]

rubberstamp LGTM.

[Hajime Morrita, 2013-10-15 09:13:23]

Committed patchset #2 manually as r159656 (presubmit successful).