Milo: Print raw json for buildbot build properties

milo/appengine/buildbot/expectations/win_chromium_rel_ng.246309.build.json

 {
   "Summary": {
     "ParentLabel": {
       "Label": "win_chromium_rel_ng",
       "URL": "."
     },
     "Label": "#246309",
     "Status": "Failure",
     "Banner": null,
     "Bot": {
@@ skipping 940 matching lines @@
           "Key": "buildername",
           "Value": "win_chromium_rel_ng"
         }
       ]
     },
     {
       "GroupName": "buildbucket",
       "Property": [
         {
           "Key": "buildbucket",
-          "Value": "https://cr-buildbucket.appspot.com/b/9008659093349989168"
+          "Value": "{\"build\": {\"bucket\": \"master.tryserver.chromium.win\", \"created_by\": \"user:sky@chromium.org\", \"created_ts\": \"1467070220037120\", \"id\": \"9008659093349989168\", \"lease_key\": \"1026332682\", \"tags\": [\"builder:win_chromium_rel_ng\", \"buildset:patch/rietveld/codereview.chromium.org/2101793003/1\", \"master:master.tryserver.chromium.win\", \"user_agent:git_cl_try\"]}}"
         },
         {
           "Key": "category",
           "Value": "git_cl_try"
         },
         {
           "Key": "issue",
           "Value": "https://codereview.chromium.org/2101793003"
         },
         {
@@ skipping 58 matching lines @@
       "RequestRevision": "",
       "Revision": "",
       "Description": "",
       "Title": "",
       "Changelist": null,
       "CommitURL": "",
       "File": []
     }
   ]
 }