add -r to DM

dm/DMJsonWriter.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef DMJsonWriter_DEFINED
 #define DMJsonWriter_DEFINED
 
@@ skipping 26 matching lines @@
 
     /**
      *  Add a Failure from a Test.
      */
     static void AddTestFailure(const skiatest::Failure&);
 
     /**
      *  Write all collected results to the file FLAGS_writePath[0]/dm.json.
      */
     static void DumpJson();
+
+    /**
+     * Read JSON file at path written by DumpJson, calling callback for each
+     * BitmapResult recorded in the file.  Return success.
+     */
+    static bool ReadJson(const char* path, void(*callback)(BitmapResult));
 };
 
+
 } // namespace DM
 #endif // DMJsonWriter_DEFINED