Make ImageBitmap support structured clone

      ImageBitmap is already transferable, but at this moment we cannot use
postMessage(ImageBitmap) via structured clone. This CL will enable it.

This CL also includes a layout test and sends an ImageBitmap to a
worker, the worker sends the ImageBitmap back to main, and does check
that the underline pixel data survives the round trip.

BUG=579539

Review URL: https://codereview.chromium.org/1616213003

Cr-Commit-Position: refs/heads/master@{#371155}