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/



Signed-off-by: Fedor Indutny <fedor@indutny.com>