Milo: Add LogDog annotation stream support.

appengine/cmd/milo/swarming/expectations/build-unicode.json

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       "Label": "Task ",
       "Img": "",
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       "Alt": ""
     },
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+    "SubLink": [
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+        "URL": "foo/stdout",
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+      }
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     "Started": "2016-03-14T10:00:01.85071Z",
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     "Text": [
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@@ skipping 70 matching lines @@
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   "PropertyGroup": [
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-    {
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         },
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         },
@@ skipping 11 matching lines @@
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       "Property": null
     },
     {
       "GroupName": "bot_update",
       "Property": null
     },
     {
       "GroupName": "Main",
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+    },
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+          "Key": "allow_milo",
+          "Value": "1"
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+      ]
     }
   ],
   "Blame": null
 }