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