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

Review URL: https://codereview.chromium.org/227653002

git-svn-id: svn://svn.chromium.org/blink/trunk@171036 bbb929c8-8fbe-4397-9dbb-9b2b20218538