Remove base::SimpleThread::SetThreadPriority() interface
Initial thread priorities should be set on thread creation.
After fixing SingleThreadTaskGraphRunner for Android, and
WASAPIAudio{In|Out}putStream for Windows, there are no
callers of base::SimpleThread::SetThreadPriority().
Since the method has a tricky restriction due to sandbox, remove
the interface. See discussion at crbug.com/468793.
BUG=468793
Review URL: https://codereview.chromium.org/
1180673004
Cr-Commit-Position: refs/heads/master@{#334553}