fix multithread related crashes in skpdiff

tools/skpdiff/SkDifferentPixelsMetric.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkDifferentPixelsMetric_DEFINED
 #define SkDifferentPixelsMetric_DEFINED
 
 #include "SkTDArray.h"
 
 #if SK_SUPPORT_OPENCL
 #include "SkCLImageDiffer.h"
 #else
 #include "SkImageDiffer.h"
 #endif
 
 /**
  * A differ that measures the percentage of different corresponding pixels. If the two images are
  * not the same size or have no pixels, the result will always be zero.
  */
 class SkDifferentPixelsMetric :
 #if SK_SUPPORT_OPENCL
     public SkCLImageDiffer {
 #else
     public SkImageDiffer {
 #endif
 public:
-    SkDifferentPixelsMetric() : fPOIAlphaMask(false) {}
-
-    virtual const char* getName() SK_OVERRIDE;
-    virtual bool enablePOIAlphaMask() SK_OVERRIDE;
-    virtual int queueDiff(SkBitmap* baseline, SkBitmap* test) SK_OVERRIDE;
-    virtual void deleteDiff(int id) SK_OVERRIDE;
-    virtual bool isFinished(int id) SK_OVERRIDE;
-    virtual double getResult(int id) SK_OVERRIDE;
-    virtual int getPointsOfInterestCount(int id) SK_OVERRIDE;
-    virtual SkIPoint* getPointsOfInterest(int id) SK_OVERRIDE;
-    virtual SkBitmap* getPointsOfInterestAlphaMask(int id) SK_OVERRIDE;
+    virtual const char* getName() const SK_OVERRIDE;
+    virtual bool diff(SkBitmap* baseline, SkBitmap* test, bool computeMask,
+                      Result* result) const SK_OVERRIDE;
 
 protected:
 #if SK_SUPPORT_OPENCL
     virtual bool onInit() SK_OVERRIDE;
 #endif
 
 private:
-    bool fPOIAlphaMask;
-
-    struct QueuedDiff;
-    SkTDArray<QueuedDiff> fQueuedDiffs;
-
 #if SK_SUPPORT_OPENCL
     cl_kernel fKernel;
 
     typedef SkCLImageDiffer INHERITED;
 #else
     typedef SkImageDiffer INHERITED;
 #endif
 };
 
 #endif