Make createImageBitmap return a promise object.

This patch introduces ImageBitmapFactories which will also be implemented in WorkerGlobalScope in a later patch.

These changes are hidden behind the experimental canvas features flag.

Spec: http://www.whatwg.org/specs/web-apps/current-work/multipage/timers.html#images

Relevant patches:
https://codereview.chromium.org/18191016/
https://codereview.chromium.org/18518004/
https://codereview.chromium.org/19771003/

BUG=261343, 166658

Review URL: https://chromiumcodereview.appspot.com/21136004

git-svn-id: svn://svn.chromium.org/blink/trunk@155383 bbb929c8-8fbe-4397-9dbb-9b2b20218538