fix multithread related crashes in skpdiff

BUG=skia:1798
R=mtklein@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/60833002

git-svn-id: http://skia.googlecode.com/svn/trunk@12252 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/skpdiff/SkCLImageDiffer.h b/tools/skpdiff/SkCLImageDiffer.h
index 032ee6f..6e9c2dc 100644
--- a/tools/skpdiff/SkCLImageDiffer.h
+++ b/tools/skpdiff/SkCLImageDiffer.h
@@ -26,7 +26,7 @@
 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
@@ -80,12 +80,15 @@
      * @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;