For sharing semaphores globally.

Bug: angleproject:4877
Change-Id: I472e0902fd04ca8350d74e6c0ae6925ee930ccf9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2319370
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>