Avoid deadlock when attempting to GC during synchronous loading in a worker.

R=haraken@chromium.org, tkent@chromium.org, vegorov@chromium.org
BUG=356307

Review URL: https://codereview.chromium.org/212553002

git-svn-id: svn://svn.chromium.org/blink/trunk@170047 bbb929c8-8fbe-4397-9dbb-9b2b20218538