Post cleanup after deprecating WorkerAllowMainThreadBridgeBase

This needs to land after chromium-side patch
(https://codereview.chromium.org/46583005/) lands.

BUG=none

Review URL: https://codereview.chromium.org/52093008

git-svn-id: svn://svn.chromium.org/blink/trunk@161325 bbb929c8-8fbe-4397-9dbb-9b2b20218538