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/
Revert "test: fix up weakref.cc after v8 api change"

No longer necessary after cherry-picking r21297 from upstream.

This reverts commit 69902c2531ec55074457841ccd195536bc17037e.