Revert of Switch to use per isolate CallCompletedCallbacks (https://codereview.chromium.org/247623006/)

Reason for revert:
Probably broke a lot of Chromium tests: http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests%20%28dbg%29/builds/2361

Original issue's description:
> Switch to use per isolate CallCompletedCallbacks
> 
> BUG=none
> R=adamk@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=172366

TBR=adamk@chromium.org,haraken@chromium.org,jochen@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=none

Review URL: https://codereview.chromium.org/246693006

git-svn-id: svn://svn.chromium.org/blink/trunk@172398 bbb929c8-8fbe-4397-9dbb-9b2b20218538