fix broken build on mac due to compiler warning

tools/skpdiff/SkImageDiffer.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 #include "SkImageDecoder.h"
 
 #include "SkImageDiffer.h"
 #include "skpdiff_util.h"
 
+const double SkImageDiffer::RESULT_CORRECT = 1.0f;
+const double SkImageDiffer::RESULT_INCORRECT = 0.0f;
 
 SkImageDiffer::SkImageDiffer()
     : fIsGood(true) {
 
 }
 
 SkImageDiffer::~SkImageDiffer() {
 
 }
 
 int SkImageDiffer::queueDiffOfFile(const char baseline[], const char test[]) {
     SkBitmap baselineBitmap;
     SkBitmap testBitmap;
     if (!SkImageDecoder::DecodeFile(baseline, &baselineBitmap)) {
         SkDebugf("Failed to load bitmap \"%s\"\n", baseline);
         return -1;
     }
     if (!SkImageDecoder::DecodeFile(test, &testBitmap)) {
         SkDebugf("Failed to load bitmap \"%s\"\n", test);
         return -1;
     }
     return this->queueDiff(&baselineBitmap, &testBitmap);
 }