Add OffscreenCanvas to ImageBitmapSource union typedef

This change makes it possible to call createImageBitmap with
an OffscreenCanvas object as an argument.  It also takes care
of propagating the origin-clean flag.

BUG=672571
TBR=haraken@chromium.org

Review-Url: https://codereview.chromium.org/2570613002
Cr-Commit-Position: refs/heads/master@{#438617}
12 files changed