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For passthrough, with extension EGL_ANGLE_display_semaphore_share_group,
semaphores are shared globally.
For GLES2Decoder, all GL contexts are in the same shared group, so
semaphores are also shared globally.
So we can reused GL semaphores cross GL contexts.

Bug: 1004772
Change-Id: I1d23cdc991b2e99d6fe70fdd0560c97dafe73e7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2319368
Commit-Queue: Peng Huang <penghuang@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793398}