GitLab

If the main thread is unblocked and we access this function on the impl thread,
we're in a racy situation. This should be an error.

BUG=235188

Review URL: https://chromiumcodereview.appspot.com/14179022

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202942 0039d316-1c4b-4281-b951-d872f2087c98