rebaseline_server: make intermediate JSON specify base urls for diff images

gm/rebaseline_server/testdata/outputs/expected/results_test.ResultsTest.test_gm/gm.json

 {
   "extraColumnHeaders": {
     "builder": {
       "headerText": "builder", 
       "isFilterable": true, 
       "isSortable": true, 
       "valuesAndCounts": {
         "Test-Android-GalaxyNexus-SGX540-Arm7-Release": 13, 
         "Test-Mac10.7-MacMini4.1-GeForce320M-x86_64-Debug": 15
       }
@@ skipping 34 matching lines @@
         "filterbitmap_checkerboard_192_192": 2, 
         "filterbitmap_checkerboard_32_2": 2, 
         "texdata": 1
       }
     }
   }, 
   "header": {
     "dataHash": "2099241024256114776", 
     "isEditable": false, 
     "isExported": true, 
-    "schemaVersion": 1, 
+    "schemaVersion": 2, 
     "timeNextUpdateAvailable": null, 
     "timeUpdated": 12345678, 
     "type": "all"
   }, 
   "imagePairs": [
     {
       "differenceData": {
         "maxDiffPerChannel": [
           128, 
           128, 
@@ skipping 468 matching lines @@
         "builder": "Test-Mac10.7-MacMini4.1-GeForce320M-x86_64-Debug", 
         "config": "pdf-poppler", 
         "resultType": "succeeded", 
         "test": "3x3bitmaprect"
       }, 
       "imageAUrl": "bitmap-64bitMD5/3x3bitmaprect/16723580409414313678.png", 
       "imageBUrl": "bitmap-64bitMD5/3x3bitmaprect/16723580409414313678.png", 
       "isDifferent": false
     }
   ], 
-  "imageSets": [
-    {
+  "imageSets": {
+    "diffs": {
+      "baseUrl": "/static/generated-images/diffs", 
+      "description": "color difference per channel"
+    }, 
+    "imageA": {
       "baseUrl": "http://chromium-skia-gm.commondatastorage.googleapis.com/gm", 
       "description": "expected image"
     }, 
-    {
+    "imageB": {
       "baseUrl": "http://chromium-skia-gm.commondatastorage.googleapis.com/gm", 
       "description": "actual image"
+    }, 
+    "whiteDiffs": {
+      "baseUrl": "/static/generated-images/whitediffs", 
+      "description": "differing pixels in white"
     }
-  ]
+  }
 }