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Previously, VideoFrameCompositor assumed that, if it had a client,
that the client would drive VideoFrame updates based on the vsync
clock.  If an external consumer (e.g., WebGL) painted the frame,
VFC would still not update the current frame, since the client would
take care of it.

VideoFrameSubmitter is a client that sometimes elides frame updates,
if it believes that no consumer is present (e.g., if the
corresponding SurfaceLayer is not in the layer tree, or is not
visible).  However, this would break external consumers, since they
would no longer get current VideoFrames.

This CL allows a VideoFrameProvider::Client to signal that it isn't
doing frame updates.  In this case, VFC will still try to move the
frame forward to match the wall clock, so that external consumers
get current frames.

Additionally, this CL causes SurfaceLayerImpl to signal that it's
no longer visible upon destruction.  This catches cases where a
video element is removed from the DOM.

Bug: 869277
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ia53a349af4add6d4792ef16e0306aa7c139ab70c
Reviewed-on: https://chromium-review.googlesource.com/1159274Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581605}