Add ability to output ImageBaseGSUrl to render_picture and use in rebaseline server

gm/rebaseline_server/imagepair_test.py

 #!/usr/bin/python
 
 """
 Copyright 2014 Google Inc.
 
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 
 Test imagepair.py
 """
@@ skipping 73 matching lines @@
             None,
             # expected output:
             {
                 'differenceData': {
                     'maxDiffPerChannel': [255, 255, 247],
                     'numDifferingPixels': 662,
                     'percentDifferingPixels': 0.0662,
                     'perceptualDifference': 0.06620300000000157,
                     'diffUrl': 'arcofzorro_16206093933823793653_png_png-vs-' +
                         'arcofzorro_13786535001616823825_png_png.png',
-                    'whiteDiffUrl': 'arcofzorro_16206093933823793653_png_png' + 
+                    'whiteDiffUrl': 'arcofzorro_16206093933823793653_png_png' +
                         '-vs-arcofzorro_13786535001616823825_png_png.png',
                 },
                 'imageAUrl': 'arcofzorro/16206093933823793653.png',
                 'imageBUrl': 'arcofzorro/13786535001616823825.png',
                 'isDifferent': True,
             },
         ],
 
         [
             # inputs:
@@ skipping 83 matching lines @@
                 'isDifferent': True,
             },
         ],
 
     ]
 
     db = imagediffdb.ImageDiffDB(self.temp_dir)
     for selftest in selftests:
       image_pair = imagepair.ImagePair(
           image_diff_db=db,
-          base_url=IMG_URL_BASE,
+          imageA_base_url=IMG_URL_BASE,
+          imageB_base_url=IMG_URL_BASE,
           imageA_relative_url=selftest[0],
           imageB_relative_url=selftest[1],
           expectations=selftest[2],
           extra_columns=selftest[3],
           download_all_images=True)
       self.assertEqual(image_pair.as_dict(), selftest[4])
 
 
 def main():
   suite = unittest.TestLoader().loadTestsFromTestCase(ImagePairTest)
   unittest.TextTestRunner(verbosity=2).run(suite)
 
 
 if __name__ == '__main__':
   main()