Add tests for CORS safety in toDataURL()

Bug: 930057
Change-Id: If4363152bb6a552fe70120b98318de41d87a4952
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1468005
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Aaron Krajeski <aaronhk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643830}