fix multithread related crashes in skpdiff

tools/skpdiff/SkPMetric.h

Index: tools/skpdiff/SkPMetric.h
diff --git a/tools/skpdiff/SkPMetric.h b/tools/skpdiff/SkPMetric.h
index 5963c2f33ca48257a99ae572d844bd9fa1c6e117..4db3e053b52cbee8d63e775a6273323a76b83e0d 100644
--- a/tools/skpdiff/SkPMetric.h
+++ b/tools/skpdiff/SkPMetric.h
@@ -19,22 +19,9 @@
 class SkPMetric : public SkImageDiffer {
 public:
     virtual const char* getName() 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 bool diff(SkBitmap* baseline, SkBitmap* test, Result* result) SK_OVERRIDE;
 
 private:
-    struct QueuedDiff {
-        bool finished;
-        double result;
-        SkTDArray<SkIPoint> poi;
-    };
-
-    SkTArray<QueuedDiff> fQueuedDiffs;
-
     typedef SkImageDiffer INHERITED;
 };