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..64df37d0f2c2edce7f8592a1811ac0e0f6a0d3c2 100644
--- a/base/synchronization/lock.h
+++ b/base/synchronization/lock.h
@@ -61,13 +61,15 @@ class BASE_EXPORT Lock {
   void AssertAcquired() const;
 #endif  // DCHECK_IS_ON()
 
+  // Whether it is safe to use a Lock from different thread priorities (i.e.
+  // there won't be priority inversion for an extended period of time).
+  static bool IsSafeWithMultipleThreadPriorities() {

[robliao, 2016/08/03 18:02:10]

I would go with HandlesMultipleThreadPriorities. Safe is too strong of a
guarantee.

[fdoray, 2016/08/03 18:34:48]

Done.

 #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();
-  }
+#else
+    return true;

[gab, 2016/08/03 18:04:57]

Add a comment about why that's true on Windows (summarize and refer to
https://msdn.microsoft.com/library/windows/desktop/ms684831.aspx)

and also 
#elif (OS_WIN) above and 
#else
#error "Unsupported platform"
#endif

[fdoray, 2016/08/03 18:34:48]

Done.

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