Introduce v8::Object::GetIsolate().

API=v8::Object::GetIsolate()
LOG=Y
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/617263003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24376 ce2b1a6d-e550-0410-aec6-3dcde31c8c00