Fix OffscreenCanvas resizing behavior

This change ensures that the width and height attributes of
a placeholder canvas reflect the size of frames committed
on the OffscreenCanvas. Also, an exception is thrown if
an attempt is made to set a size attribute on a placeholder
canvas.

BUG=754733, 754736

TBR=zmo@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;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
Change-Id: I6ec4296df5f8d47e22753e40e91eee616a3f9399
Reviewed-on: https://chromium-review.googlesource.com/612513
Reviewed-by: Justin Novosad <junov@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496120}
10 files changed