Add methods to allow resuming execution after calling TerminateExecution().

Add methods to allow resuming execution after calling TerminateExecution().

Two new methods are added to allow embedders to determine that execution
should be resumed at a particular point in the stack without being forced
to unwind all JS frames.

* V8::CancelTerminateExecution() -- When execution is terminated via a call
  to V8::TerminateExecution(), this method can be called to clear the
  termination exception so that the engine can continue to be used.

* TryCatch::HasTerminated() -- When a TryCatch has caught a termination
  exception, HasTerminated() will return true to indicate it is valid to
  call V8::ResumeExecution() if desired.

A test case is added to cctest/test-thread-termination.cc.

BUG=v8:2361

Patch from Andrew Paprocki <andrew@ishiboo.com>.

Review URL: https://chromiumcodereview.appspot.com/11142013
Patch from Andrew Paprocki <andrew@ishiboo.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14022 ce2b1a6d-e550-0410-aec6-3dcde31c8c00