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The previous attempt to land this CL was reverted because an early call
to PlatformThread::SetCurrentThreadPriority() in headless caused the
FeatureList to be accessed before it was initialized. This CL avoids
accessing an uninitialized FeatureList if the target priority is not
BACKGROUND.

Diff between ps 1 and 5 shows changes made since revert.

Previous attempts to land:
  https://chromium-review.googlesource.com/1171482
  https://chromium-review.googlesource.com/1185529
  https://chromium-review.googlesource.com/1190842

NOTE: If this causes new test failures, we'll add the value returned
by GetThreadPriority() to the switch/case of allowed values.

This CL adds a feature to use THREAD_MODE_BACKGROUND_BEGIN instead of
THREAD_PRIORITY_LOWEST for ThreadPriority::BACKGROUND threads. This
lowers the disk and network I/O priority of the thread in addition to
the CPU scheduling priority. MSDN recommends using this setting for
threads that perform background work.
https://docs.microsoft.com/en-us/windows/desktop/api/processthreadsapi/nf-processthreadsapi-setthreadpriority

Bug: 872820
Change-Id: I2c9f17390eaaba2f8f454faa29836bbc37e3d47c
Reviewed-on: https://chromium-review.googlesource.com/1195677
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587349}