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We drop GL contexts on low-end devices when we go to the background.
This is currently initiated by the GPU process, which kills all GPU
channels, eventually causing context-loss in the various clients.

While the current approach works, it makes it difficult to avoid
rendering artifacts on app resume. As context-loss is lazily
identified, it's possible for the browser and renderer to be notified
at very different times. This can lead to races where the browser will
attempt to draw with invalid renderer resources.

To avoid these issues, this CL causes the context-dropping to be
triggered from the browser itself. This allows the browser to drop
renderer resources at the same time, greatly reducing (but
unfortunately not eliminating) the possibility of a race.

If we still see these issues with any frequency, further follow-up
work, leveraging Surface Synchronization, will likely be able to
fully eliminate these cases.

Bug: 792120
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;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: I1c0f2c6000fc9c1f4171952b09484101a220654c
Reviewed-on: https://chromium-review.googlesource.com/959605Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545879}