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Change metal_util::TestShader to test either compile or link, and
change the default behavior to link.

Ensure that the Metal calls for the test are invoked on a thread
that has base::TaskPriority::HIGHEST.

Bug: 974219
Change-Id: I6d2db6c40182286a4af3489968a995dca73e7944
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2024922Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736148}