Revert of Modernize SkMutex and SkSemaphore.

include/core/SkPixelRef.h

Index: include/core/SkPixelRef.h
diff --git a/include/core/SkPixelRef.h b/include/core/SkPixelRef.h
index d836b2fd7c31deb407632c15eb58dad74bdf6ccd..90e0de540d14c7205fcdf3facdd4c8eeff98c59f 100644
--- a/include/core/SkPixelRef.h
+++ b/include/core/SkPixelRef.h
@@ -342,7 +342,7 @@
     /** Return the mutex associated with this pixelref. This value is assigned
         in the constructor, and cannot change during the lifetime of the object.
     */
-    SkMutex* mutex() const { return &fMutex; }
+    SkBaseMutex* mutex() const { return &fMutex; }
 
     // only call from constructor. Flags this to always be locked, removing
     // the need to grab the mutex and call onLockPixels/onUnlockPixels.