WebRTC: Add flakiness dashboard reporting.

scripts/slave/recipes/webrtc/chromium.expected/chromium_webrtc_fyi_WinXP_Tester.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 209 matching lines @@
       "-u",
       "[BUILD]\\scripts\\slave\\runtest.py",
       "--target",
       "Release",
       "--factory-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"buildername\": \"WinXP Tester\", \"buildnumber\": 571, \"mastername\": \"chromium.webrtc.fyi\", \"parent_buildername\": \"Win Builder\", \"parent_got_revision\": \"12345\", \"recipe\": \"webrtc/chromium\", \"revision\": \"12345\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--build-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"buildername\": \"WinXP Tester\", \"buildnumber\": 571, \"mastername\": \"chromium.webrtc.fyi\", \"parent_buildername\": \"Win Builder\", \"parent_got_revision\": \"12345\", \"recipe\": \"webrtc/chromium\", \"revision\": \"12345\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--annotate=gtest",
       "--test-type=content_unittests",
+      "--generate-json-file",
+      "-o",
+      "gtest-results/content_unittests.exe",
       "--builder-name=WinXP Tester",
       "--slave-name=TestSlavename",
       "--build-number=571",
       "content_unittests.exe",
       "--gtest_filter=WebRtc*:WebRTC*:RTC*:MediaStream*"
     ],
     "name": "content_unittests"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\process_dumps.py",
       "--target",
       "Release"
     ],
     "name": "process_dumps"
   }
 ]