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This changes streams to only propagate inherited prioirities to
already existing wait fences.

New wait fences initially inherit the default priority of the waiter.
If the waiter gets a priority bump, that priority is propagated to
existing wait fences at that time.

BUG=786820

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: Iab04f21985e69aba7f3b90b7ffe2545d3c8510e6
Reviewed-on: https://chromium-review.googlesource.com/778422
Commit-Queue: Victor Miura <vmiura@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518453}