rebaseline_server: include setA/setB descriptions in output JSON

gm/rebaseline_server/testdata/outputs/expected/compare_configs_test.CompareConfigsTest.test_gm/gm.json

 {
   "extraColumnHeaders": {
     "builder": {
       "headerText": "builder", 
       "isFilterable": true, 
       "isSortable": true, 
       "useFreeformFilter": false, 
       "valuesAndCounts": [
         [
           "Test-Android-GalaxyNexus-SGX540-Arm7-Release", 
@@ skipping 89 matching lines @@
   "extraColumnOrder": [
     "builder", 
     "config", 
     "resultType", 
     "test"
   ], 
   "header": {
     "dataHash": "-5829724510169924592", 
     "isEditable": false, 
     "isExported": true, 
-    "schemaVersion": 4, 
+    "schemaVersion": 5, 
     "timeNextUpdateAvailable": null, 
     "timeUpdated": 12345678, 
     "type": "all"
   }, 
   "imagePairs": [
     {
       "extraColumns": {
         "builder": "Test-Android-GalaxyNexus-SGX540-Arm7-Release", 
         "config": "TODO", 
         "resultType": "no-comparison", 
@@ skipping 199 matching lines @@
     "imageB": {
       "baseUrl": "http://chromium-skia-gm.commondatastorage.googleapis.com/gm", 
       "description": "gpu"
     }, 
     "whiteDiffs": {
       "baseUrl": "/static/generated-images/whitediffs", 
       "description": "differing pixels in white"
     }
   }
 }