Enable WebGL rendering into ImageCHROMIUM, backed by a scanout buffer to enable overlay support.

BUG=372449

Review URL: https://codereview.chromium.org/274833002

git-svn-id: svn://svn.chromium.org/blink/trunk@174402 bbb929c8-8fbe-4397-9dbb-9b2b20218538