Expose createImageBitmap in worker threads.

R=junov@chromium.org,senorblanco@chromium.org
BUG=269517,166658

Review URL: https://chromiumcodereview.appspot.com/22827005

git-svn-id: svn://svn.chromium.org/blink/trunk@156181 bbb929c8-8fbe-4397-9dbb-9b2b20218538