Make OffscreenCanvas commit() propagate the origin-clean flag.

This change block user code from accessing cross-origin image data
from a placeholder canvas. The placeholder code was modified to
use StaticBitmapImage instead of Image to gain access to the
originClean() method without downcasting.

BUG=673348

Review-Url: https://codereview.chromium.org/2566313002
Cr-Commit-Position: refs/heads/master@{#438172}
6 files changed