Add API for adding and removing CallCompletedCallbacks to Isolate

The API currently just forwards to the global methods. A follow-up
change will move the callback handling to the Isolate and deprecate the
global versions.

BUG=
R=dcarney@chromium.org, svenpanne@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/215893005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20463 ce2b1a6d-e550-0410-aec6-3dcde31c8c00