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This reverts commit 16307825.

Reason for revert: crashes in tests on Mac Release (Intel) - see attached bug for information

Bug: 1082966

Original change's description:
> Metal: Run test shaders on a ThreadPriority::DISPLAY thread
> 
> The Metal test shaders are compiled in a function that comes from
> a PostTask.  There is currently no path through which a PostTask
> will end up running on a ThreadPriority::DISPLAY thread.
> 
> Change the Metal test code to create a ThreadPriority::DISPLAY
> thread and do its work there. A separate change is making that
> correspond to a pthread priority of 50.
> 
> Of note is that Metal's callback to Chrome comes from a Metal-created
> thread which has priority 31.
> 
> Also change the default test shader to be a Compile instead of a
> Link (the Link tests have not been completed).
> 
> Bug: 974219
> Change-Id: Ieb5ab0748b3aa255ee7dd3348870ddf812caa682
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2200357
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Commit-Queue: ccameron <ccameron@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#768962}

TBR=avi@chromium.org,ccameron@chromium.org

Change-Id: I852346697048fc916711ea60dd050b646a53705c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 974219
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2203257Reviewed-by: Dana Fried <dfried@google.com>
Cr-Commit-Position: refs/heads/master@{#768986}