fix multithread related crashes in skpdiff

tools/skpdiff/SkCLImageDiffer.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 SkCLImageDiffer_DEFINED
 #define SkCLImageDiffer_DEFINED
 
 #if SK_BUILD_FOR_MAC
 #   include <OpenCL/cl.h>
 #else
 #   include <CL/cl.h>
 #endif
 #include "SkTDArray.h"
 
 #include "SkImageDiffer.h"
 
 class SkStream;
 
 /**
  * An SkImageDiffer that requires initialization with an OpenCL device and context.
  */
 class SkCLImageDiffer : public SkImageDiffer {
 public:
     SkCLImageDiffer();
 
-    virtual bool requiresOpenCL() SK_OVERRIDE { return true; }
+    virtual bool requiresOpenCL() const SK_OVERRIDE { return true; }
 
     /**
      * Initializes the OpenCL resources this differ needs to work
      * @param  device  An OpenCL device
      * @param  context An OpenCL context of the given device
      * @return         True on success, false otherwise
      */
     virtual bool init(cl_device_id device, cl_context context);
 
 protected:
@@ skipping 33 matching lines @@
      */
     bool loadKernelSource(const char source[], const char name[], cl_kernel* kernel);
 
     /**
      * Loads a read only copy of the given bitmap into device memory and returns the block of
      * memory. This only works after init is called.
      * @param  bitmap The bitmap to load into memory
      * @param  image  A pointer to return the allocated image to
      * @return        True on success, false otherwise
      */
-    bool makeImage2D(SkBitmap* bitmap, cl_mem* image);
+    bool makeImage2D(SkBitmap* bitmap, cl_mem* image) const;
 
     cl_device_id     fDevice;
     cl_context       fContext;
     cl_command_queue fCommandQueue;
 
+protected:
+    bool fIsGood;
+
 private:
 
     typedef SkImageDiffer INHERITED;
 };
 
 #endif