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Mutex priority inheritance is useful to avoid cases where a
high-priority thread is waiting for a mutex that is held by a low
priority thread that may not get to run for a while. This is
particularly useful for realtime applications. For more discussion, see
https://www.kernel.org/doc/Documentation/pi-futex.txt

Some platforms (eg Chromecast) are known to fulfill the requirements for
safe use of mutex priority inheritance; add a GN arg and buildflag so
that priority inheritance can be enabled for those platforms.

Chromecast uses Linux 4.1 or greater, and glibc 2.23-r10, so the bugs
mentioned in LockImpl::PriorityInheritanceAvailable() are not present.

Bug: 
Change-Id: I21cecd2144d82f038e35aa30a2a51c35d541c644
Reviewed-on: https://chromium-review.googlesource.com/847642
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526884}