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

gm/rebaseline_server/testdata/outputs/expected/compare_rendered_pictures_test.CompareRenderedPicturesTest.test_endToEnd_withImageBaseGSUrl/compare_rendered_pictures.json

 {
   "extraColumnHeaders": {
     "builderA": {
       "headerText": "builderA", 
       "isFilterable": true, 
       "isSortable": true, 
       "useFreeformFilter": false, 
       "valuesAndCounts": [
         [
           null, 
@@ skipping 204 matching lines @@
       "isDifferent": false, 
       "sourceJsonFile": "./summary.json"
     }
   ], 
   "imageSets": {
     "diffs": {
       "baseUrl": "/static/generated-images/diffs", 
       "description": "color difference per channel"
     }, 
     "imageA": {
-      "baseUrl": "http://storage.cloud.google.com/fakebucket/fake/path", 
+      "baseUrl": "http://storage.cloud.google.com/superman/kent-camera/pictures", 
       "description": "setA"
     }, 
     "imageB": {
-      "baseUrl": "http://storage.cloud.google.com/fakebucket/fake/path", 
+      "baseUrl": "http://storage.cloud.google.com/batman/batarang/pictures", 
       "description": "setB"
     }, 
     "whiteDiffs": {
       "baseUrl": "/static/generated-images/whitediffs", 
       "description": "differing pixels in white"
     }
   }
 }