Synchronize OffscreenCanvas content with the placeholder canvas

This change solves the problem of propagating frames committed by
an OffscreenCanvas all the way to the placeholder canvas, whether it
be in the same script context or not.

This allows the placeholder canvas to be used as an image source,
and to be printed.

BUG=655278
TBR=kbr@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2493673002
Cr-Commit-Position: refs/heads/master@{#431953}
27 files changed