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Previously landed as
https://chromium-review.googlesource.com/c/1318438.
Reverted because
PlatformThreadWinTest.SetBackgroundThreadModeFailsInIdlePriorityProcess
failed on Windows 7. This reland adds more possible return values for
::GetThreadPriority() on Windows 7 (see diff between ps1 and ps7).

Previously, SetCurrentThreadPriority(BACKGROUND) did not affect CPU
priority on Windows in a IDLE process when the
"WindowsThreadModeBackground" feature was enabled. It only affected I/O
and memory priorities.

With this CL, a second call to ::SetThreadPriority() is made to
ensure that CPU priority is affected.

TBR=gab@chromium.org

Bug: 901483
Change-Id: I21ebc6465b3b8d10a956714d55ef88b28bf89fff
Reviewed-on: https://chromium-review.googlesource.com/c/1326423Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607269}