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This is a reland of dcb738f9

Adds a definition of RaiseProcessToHighPriority() that went missing
when launch_posix.cc stoped being linked.

Original change's description:
> Reimplement GetAppOutput and friends on Mac with posix_spawn.
>
> Mac no longer uses launch_posix.cc and all process creation goes through
> posix_spawn now.
>
> Bug: 179923
> Change-Id: I5206cace69ea7863a72b58e1a59f2465492d4cbd
> Reviewed-on: https://chromium-review.googlesource.com/c/1313334
> Reviewed-by: Mark Mentovai <mark@chromium.org>
> Commit-Queue: Robert Sesek <rsesek@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#604982}

Tbr: mark@chromium.org
Bug: 179923
Change-Id: I196f406a19a9cbc5f0e29a352062abb8e87638f0
Reviewed-on: https://chromium-review.googlesource.com/c/1315947Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605064}