Disable backpressure in ImageTransportSurfaceOverlayMac

Two changes
- Remove all use of GLFence in backpressure
  - In case GLFence objects were responsible for crashiness
- Move the glFlush from SwapBuffers to ScheduleOverlay
  - ScheduleOverlay is a regular GL call, so it may be executed
    in a more "safe" context than SwapBuffers
  - This results in many more glFlush calls, but almost all are
    no-ops

Possible outcomes
- Crashes just change signature to ScheduleCALayer
  - Not very helpful
- Crashes are reduced
  - Tells us that either
  - Our GLFence use was the cause of crashes
  - Making GL calls from inside SwapBuffers caused crashes

TBR=kbr

Bug: 863817
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;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I23342b190a0d8b652dde8f96f1cb1169b577764c
Reviewed-on: https://chromium-review.googlesource.com/1150569
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578117}
2 files changed