rebaseline_server: include setA/setB descriptions in output JSON

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

 {
   "extraColumnHeaders": {
     "resultType": {
       "headerText": "resultType", 
       "isFilterable": true, 
       "isSortable": true, 
       "useFreeformFilter": false, 
       "valuesAndCounts": [
         [
           "failed", 
@@ skipping 61 matching lines @@
   "extraColumnOrder": [
     "resultType", 
     "sourceSkpFile", 
     "tiledOrWhole", 
     "tilenum"
   ], 
   "header": {
     "dataHash": "-4754972663365911725", 
     "isEditable": false, 
     "isExported": true, 
-    "schemaVersion": 4, 
+    "schemaVersion": 5, 
+    "setA": {
+      "dir": [
+        "before-patch-fake-dir"
+      ], 
+      "repoRevision": null, 
+      "section": "actual-results"
+    }, 
+    "setB": {
+      "dir": [
+        "after-patch-fake-dir"
+      ], 
+      "repoRevision": null, 
+      "section": "actual-results"
+    }, 
     "timeNextUpdateAvailable": null, 
     "timeUpdated": 12345678, 
     "type": "all"
   }, 
   "imagePairs": [
     {
       "extraColumns": {
         "resultType": "failed", 
         "sourceSkpFile": "changed.skp", 
         "tiledOrWhole": "whole", 
@@ skipping 49 matching lines @@
     "imageB": {
       "baseUrl": "http://storage.cloud.google.com/fakebucket/fake/path", 
       "description": "setB"
     }, 
     "whiteDiffs": {
       "baseUrl": "/static/generated-images/whitediffs", 
       "description": "differing pixels in white"
     }
   }
 }