Add Lock::HandlesMultipleThreadPriorities().

base/synchronization/lock.h

Index: base/synchronization/lock.h
diff --git a/base/synchronization/lock.h b/base/synchronization/lock.h
index c2680cbcc8a145be5ce3556a76e071e595997d7a..b8dd971f30571fa5129c844c10f4634285723f68 100644
--- a/base/synchronization/lock.h
+++ b/base/synchronization/lock.h
@@ -61,13 +61,23 @@ class BASE_EXPORT Lock {
   void AssertAcquired() const;
 #endif  // DCHECK_IS_ON()
 
+  // Whether Lock mitigates priority inversion when used from different thread
+  // priorities. Priority inversion is mitigated by allowing a thread that holds
+  // a Lock to run when higher priority threads try to acquire it. Note that a
+  // low priority thread can still prevent high priority threads to run for an
+  // unbounded amount of time by holding a Lock.

[gab, 2016/08/03 18:47:25]

Hmmm? Unbounded? The whole point here I thought was that when we return true,
low priority threads *will* be readied in a reasonable amount of time?

If you meant a low priority thread holding the lock while it's active, then I
don't think this comment is necessary as that's true for threads of any priority
-- the property at stake here handles readying threads holding the blocking
resource, everything else is business as usual with Locks.

[robliao, 2016/08/03 19:17:15]

It will be readied, but if it's doing a really long I/O op, the higher priority
thread is still blocked. 

Granted this is true regardless of lock thread priority support.

[fdoray, 2016/08/03 19:24:09]

Done. Removed last sentence because it is true regardless of whether they are
used from multiple priorities.

+  static bool HandlesMultipleThreadPriorities() {
 #if defined(OS_POSIX)
-  // Whether this platform has priority inheritance available. All locks will
-  // attempt to use the priority inheritance version if available.
-  static bool PriorityInheritanceAvailable() {
     return internal::LockImpl::PriorityInheritanceAvailable();
-  }
+#elif defined(OS_WIN)
+    // Windows mitigates priority inversion by randomly boosting the priority of
+    // ready threads.
+    // https://msdn.microsoft.com/library/windows/desktop/ms684831.aspx
+    return true;
+#else
+    #error "Unsupported platform"

[gab, 2016/08/03 18:47:25]

nit: no indent for preprocessor commands

[robliao, 2016/08/03 19:17:15]

Maybe "Unsupported platform. Must specify if the lock handles multiple thread
priorities."

[fdoray, 2016/08/03 19:24:09]

Done.

[fdoray, 2016/08/03 19:24:09]

Done.

 #endif
+  }
 
 #if defined(OS_POSIX) || defined(OS_WIN)
   // Both Windows and POSIX implementations of ConditionVariable need to be