Revert of Remove SkThread.h, include SkMutex.h or SkAtomics.h as appropriate.

src/core/SkPixelRef.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmapCache.h"
-#include "SkMutex.h"
 #include "SkPixelRef.h"
+#include "SkThread.h"
 #include "SkTraceEvent.h"
 
 //#define SK_SUPPORT_LEGACY_UNBALANCED_PIXELREF_LOCKCOUNT
 //#define SK_TRACE_PIXELREF_LIFETIME
 
 #ifdef SK_BUILD_FOR_WIN32
     // We don't have SK_BASE_MUTEX_INIT on Windows.
 
     // must be a power-of-2. undef to just use 1 mutex
     #define PIXELREF_MUTEX_RING_COUNT       32
@@ skipping 366 matching lines @@
     }
 
     result->fUnlockProc = unlock_legacy_result;
     result->fUnlockContext = SkRef(this);   // this is balanced in our fUnlockProc
     result->fCTable = fRec.fColorTable;
     result->fPixels = fRec.fPixels;
     result->fRowBytes = fRec.fRowBytes;
     result->fSize.set(fInfo.width(), fInfo.height());
     return true;
 }