add initial support ofr multi-threaded execution

tools/skpdiff/SkDiffContext.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 SkDiffContext_DEFINED
 #define SkDiffContext_DEFINED
 
 #include "SkString.h"
 #include "SkTArray.h"
 #include "SkTDArray.h"
 
 class SkWStream;
 class SkImageDiffer;
 
 /**
  * Collects records of diffs and outputs them as JSON.
  */
 class SkDiffContext {
 public:
     SkDiffContext();
     ~SkDiffContext();
 
+    void setThreadCount(int threadCount) { fThreadCount = threadCount; }
+
     /**
      * Sets the differs to be used in each diff. Already started diffs will not retroactively use
      * these.
      * @param differs An array of differs to use. The array is copied, but not the differs
      *                themselves.
      */
     void setDiffers(const SkTDArray<SkImageDiffer*>& differs);
 
     /**
      * Compares two directories of images with the given differ
@@ skipping 61 matching lines @@
 
 
 private:
     struct DiffData {
         const char* fDiffName;
         double fResult;
         SkTDArray<SkIPoint> fPointsOfInterest;
     };
 
     struct DiffRecord {
+        SkString           fDifferencePath;
         SkString           fBaselinePath;
         SkString               fTestPath;
         SkTArray<DiffData>        fDiffs;
         DiffRecord*                fNext;
     };
 
     // We use linked list for the records so that their pointers remain stable. A resizable array
     // might change its pointers, which would make it harder for async diffs to record their
     // results.
     DiffRecord * fRecords;
 
     SkImageDiffer** fDiffers;
     int fDifferCount;
+    int fThreadCount;
 };
 
 #endif