Make OffscreenCanvas a member of CanvasImageSource

This change makes it possible to call the drawImage and createPattern
methods of CanvasRenderingContext2D with an OffscreenCAnvas as an
argument

BUG=632015
TBR=haraken@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2294383002
Cr-Commit-Position: refs/heads/master@{#415942}
25 files changed