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..27a2b02c95407eff3465cd6f12c9098e1c759e0e 100644
--- a/base/synchronization/lock.h
+++ b/base/synchronization/lock.h
@@ -61,13 +61,21 @@ class BASE_EXPORT Lock {
   void AssertAcquired() const;
 #endif  // DCHECK_IS_ON()
 
+  // Whether Lock mitigates priority inversion when used from different thread

[robliao, 2016/08/03 19:30:07]

One more nit: Since we're generalizing this, I would rephrase the comment as
follows.

Whether Lock makes special accommodation under contention when used with threads
of different priorities, including but not limited to priority inheritance of
the lock holder from the lock waiter of a higher priority.

[fdoray, 2016/08/03 19:32:10]

Done.

+  // priorities. Priority inversion is mitigated by allowing a thread that holds
+  // a Lock to run when higher priority threads try to acquire it.

[gab, 2016/08/03 19:36:25]

I think the second sentence is specific to the implementation (i.e. this sounds
like something that should go in the OS_POSIX block and the OS_WIN block already
has its comment)

[fdoray, 2016/08/03 19:46:25]

Done.

+  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 Must specify if the lock handles multiple thread priorities.

[gab, 2016/08/03 19:36:25]

#error Unsupported platform

is sufficient IMO

[fdoray, 2016/08/03 19:46:25]

Done.

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