Milo: Add LogDog annotation stream support.

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

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@@ skipping 14 matching lines @@
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     "Started": "2016-03-14T10:00:01.85071Z",
     "Finished": "2016-03-14T11:00:18.48602Z",
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@@ skipping 44 matching lines @@
       "Duration": 1000000000,
       "Type": "Step",
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       "Text": [
         "running recipe: \"v8\""
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   ],
   "PropertyGroup": [
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     {
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@@ skipping 56 matching lines @@
         },
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           "Key": "user_agent",
           "Value": "luci-cron"
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