make OpenCL optional for skpdiff

experimental/skpdiff/SkImageDiffer.h

Index: experimental/skpdiff/SkImageDiffer.h
diff --git a/experimental/skpdiff/SkImageDiffer.h b/experimental/skpdiff/SkImageDiffer.h
index c06538df1766f769d99f786f27aea0303493d178..6c570cbb8e41bc3d8437937047f55396cd262258 100644
--- a/experimental/skpdiff/SkImageDiffer.h
+++ b/experimental/skpdiff/SkImageDiffer.h
@@ -32,6 +32,11 @@ public:
     bool isGood() { return fIsGood; }
 
     /**
+     * Gets if this differ needs to be initialized with and OpenCL device and context.
+     */
+    virtual bool requiresOpenCL() { return false; }
+
+    /**
      * Wraps a call to queueDiff by loading the given filenames into SkBitmaps
      * @param  baseline The file path of the baseline image
      * @param  test     The file path of the test image