rebaseline_server: allow users to generate SKP diff patches on a shared instance

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

 {
   "extraColumnHeaders": {
     "builderA": {
       "headerText": "builderA", 
       "isFilterable": true, 
       "isSortable": true, 
       "useFreeformFilter": false, 
       "valuesAndCounts": [
         [
           null, 
@@ skipping 110 matching lines @@
     "resultType", 
     "sourceSkpFile", 
     "tiledOrWhole", 
     "tilenum", 
     "builderA", 
     "renderModeA", 
     "builderB", 
     "renderModeB"
   ], 
   "header": {
-    "dataHash": "-1510211866509185075", 
+    "dataHash": "-5707186260478709107", 
     "isEditable": false, 
     "isExported": true, 
     "schemaVersion": 5, 
     "setA": {
       "dir": [
         "before-patch-fake-dir"
       ], 
       "repoRevision": null, 
       "section": "actual-results"
     }, 
@@ skipping 15 matching lines @@
         "builderB": null, 
         "renderModeA": null, 
         "renderModeB": null, 
         "resultType": "failed", 
         "sourceSkpFile": "changed.skp", 
         "tiledOrWhole": "whole", 
         "tilenum": "N/A"
       }, 
       "imageAUrl": "changed_skp/bitmap-64bitMD5_3101044995537104462.png", 
       "imageBUrl": "changed_skp/bitmap-64bitMD5_13623922271964399662.png", 
-      "isDifferent": true
+      "isDifferent": true, 
+      "sourceJsonFile": "./summary.json"
     }, 
     {
       "extraColumns": {
         "builderA": null, 
         "builderB": null, 
         "renderModeA": null, 
         "renderModeB": null, 
         "resultType": "no-comparison", 
         "sourceSkpFile": "only-in-after.skp", 
         "tiledOrWhole": "whole", 
         "tilenum": "N/A"
       }, 
       "imageAUrl": null, 
       "imageBUrl": "only-in-after_skp/bitmap-64bitMD5_2320185040577047131.png", 
-      "isDifferent": true
+      "isDifferent": true, 
+      "sourceJsonFile": "./summary.json"
     }, 
     {
       "extraColumns": {
         "builderA": null, 
         "builderB": null, 
         "renderModeA": null, 
         "renderModeB": null, 
         "resultType": "no-comparison", 
         "sourceSkpFile": "only-in-before.skp", 
         "tiledOrWhole": "whole", 
         "tilenum": "N/A"
       }, 
       "imageAUrl": "only-in-before_skp/bitmap-64bitMD5_2320185040577047131.png", 
       "imageBUrl": null, 
-      "isDifferent": true
+      "isDifferent": true, 
+      "sourceJsonFile": "./summary.json"
     }, 
     {
       "extraColumns": {
         "builderA": null, 
         "builderB": null, 
         "renderModeA": null, 
         "renderModeB": null, 
         "resultType": "succeeded", 
         "sourceSkpFile": "unchanged.skp", 
         "tiledOrWhole": "whole", 
         "tilenum": "N/A"
       }, 
       "imageAUrl": "unchanged_skp/bitmap-64bitMD5_3322248763049618493.png", 
       "imageBUrl": "unchanged_skp/bitmap-64bitMD5_3322248763049618493.png", 
-      "isDifferent": false
+      "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", 
       "description": "setA"
     }, 
     "imageB": {
       "baseUrl": "http://storage.cloud.google.com/fakebucket/fake/path", 
       "description": "setB"
     }, 
     "whiteDiffs": {
       "baseUrl": "/static/generated-images/whitediffs", 
       "description": "differing pixels in white"
     }
   }
 }