fix multithread related crashes in skpdiff

tools/skpdiff/SkDifferentPixelsMetric.h

Index: tools/skpdiff/SkDifferentPixelsMetric.h
diff --git a/tools/skpdiff/SkDifferentPixelsMetric.h b/tools/skpdiff/SkDifferentPixelsMetric.h
index 614f92035672b214ac7b886449fca6742acf4eb3..06c56b1cca245bf6c86046530d2657ee45c80d73 100644
--- a/tools/skpdiff/SkDifferentPixelsMetric.h
+++ b/tools/skpdiff/SkDifferentPixelsMetric.h
@@ -27,17 +27,9 @@ class SkDifferentPixelsMetric :
     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
@@ -45,11 +37,6 @@ protected:
 #endif
 
 private:
-    bool fPOIAlphaMask;
-
-    struct QueuedDiff;
-    SkTDArray<QueuedDiff> fQueuedDiffs;
-
 #if SK_SUPPORT_OPENCL
     cl_kernel fKernel;