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}

base/synchronization/lock.h

@@ -61,13 +61,22 @@ class BASE_EXPORT Lock {
   void AssertAcquired() const;
 #endif  // DCHECK_IS_ON()
 
+  // Whether Lock mitigates priority inversion when used from different thread
+  // 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() {
+    // POSIX mitigates priority inversion by setting the priority of a thread
+    // holding a Lock to the maximum priority of any other thread waiting on it.
     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
 #endif
+  }
 
 #if defined(OS_POSIX) || defined(OS_WIN)
   // Both Windows and POSIX implementations of ConditionVariable need to be