Implementing promise-based commit for driving OffscreenCanvas animations

This is an experimental prototype API for demonstrating a proposal to use
promises instead of requestAnimationFrame for driving animations.  This
initial implementation uses a delayed task to simulate the BeginFrame
signal.  Properly hooking-up BeginFrame will be solved in a follow-up
code change.

This API was discussed here:
https://github.com/whatwg/html/issues/2139

BUG=676131
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/2594843002
Cr-Commit-Position: refs/heads/master@{#441711}
21 files changed