Revert of Hook up gin platform to blink, and explicitly manage the main thread isolate (https://codereview.chromium.org/227653002/)

Reason for revert:
Broke browser_tests on Linux: http://build.chromium.org/p/chromium.webkit/builders/Linux%20Tests/builds/35802


Original issue's description:
> Hook up gin platform to blink, and explicitly manage the main thread isolate
> 
> We'll soon get rid of the v8 default isolate, so we'll need to manage
> our own.
> 
> To test the platform implementation, run with
>   --js-flags="--noparallel_sweeping --concurrent-sweeping --job-based-sweeping"
> 
> BUG=359977
> R=dcarney@chromium.org,haraken@chromium.org
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=171036

TBR=dcarney@chromium.org,haraken@chromium.org,jochen@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=359977

Review URL: https://codereview.chromium.org/225023038

git-svn-id: svn://svn.chromium.org/blink/trunk@171053 bbb929c8-8fbe-4397-9dbb-9b2b20218538