deps: cherry-pick r21297 from v8 trunk

Changes the return value of PropertyCallbackInfo<T>::This() from
Local<Value> back to Local<Object>.  See [1] and [2] for background.

[1] https://groups.google.com/forum/#!topic/v8-users/wP2UcQ4cBW4
[2] https://codereview.chromium.org/285643008/
deps: cherry-pick r21466 from v8 trunk

Check for cached transition to ExternalArray elements kind.
See [1] and [2] for details.

[1] https://code.google.com/p/v8/issues/detail?id=3337
[2] https://codereview.chromium.org/291193011
src: fix up after v8 upgrade

Fix up the vm watchdog, V8::TerminateExecution() is isolate-ified now.