See doc/PresentSemaphores.md for details.

Bug: angleproject:3450
Bug: angleproject:3670
Change-Id: I52d5bd13a4af25f224d386c9584525c182af6f17
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1776880Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>