Reland r172366 - Switch to use per isolate CallCompletedCallbacks"

Don't install the callbacks during unit tests.

> Switch to use per isolate CallCompletedCallbacks
>
> BUG=none
> R=adamk@chromium.org
>
> Review URL: https://codereview.chromium.org/247623006/

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/256523002

git-svn-id: svn://svn.chromium.org/blink/trunk@172474 bbb929c8-8fbe-4397-9dbb-9b2b20218538