The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/206071838287c145e6a71f57744fd47bf24d614b

commit 206071838287c145e6a71f57744fd47bf24d614b
Author: robliao <robliao@chromium.org>
Date: Mon Aug 01 20:59:42 2016

Add PTHREAD_PRIO_INHERIT Locks for Mac

The Browser Task Scheduler will be using thread priorities.
PTHREAD_PRIO_INHERIT will help avoid priority inversion with the use of thread
priorities on Mac.

Linux currently cannot use them due to security concerns in the kernel priority
inheritance futex.

BUG= 611856 

Review-Url: https://codereview.chromium.org/2178503003
Cr-Commit-Position: refs/heads/master@{#409052}

[modify] https://crrev.com/206071838287c145e6a71f57744fd47bf24d614b/base/synchronization/lock.h
[modify] https://crrev.com/206071838287c145e6a71f57744fd47bf24d614b/base/synchronization/lock_impl.h
[modify] https://crrev.com/206071838287c145e6a71f57744fd47bf24d614b/base/synchronization/lock_impl_posix.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d84dc069a147eb0209b9f7713beafb3de0dec26b

commit d84dc069a147eb0209b9f7713beafb3de0dec26b
Author: fdoray <fdoray@chromium.org>
Date: Thu Aug 04 20:36:26 2016

Add Lock::HandlesMultipleThreadPriorities().

This static method is available on all platform. It replaces
Lock::PriorityInheritanceAvailable(). It will simplify code
that checks whether priority inversion will be mitigated when
Lock is used from different thread priorities.

E.g.:

  if (Lock::Lock::PriorityInheritanceAvailable())
    PlatformThread::SetCurrentThreadPriority(...);

Becomes:

  if (Lock::HandlesMultipleThreadPriorities())
    PlatformThread::SetCurrentThreadPriority(...);

BUG= 611856 

Review-Url: https://codereview.chromium.org/2206263002
Cr-Commit-Position: refs/heads/master@{#409890}

[modify] https://crrev.com/d84dc069a147eb0209b9f7713beafb3de0dec26b/base/synchronization/lock.h

Works as much as possible (i.e. on Mac -- see PriorityInheritanceAvailable() for details).