WebRTC: Add flakiness dashboard reporting.

scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_android_tests__kk_nexus5__failing_test.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "client.webrtc",
       "--builder",
       "Android Tests (KK Nexus5)",
@@ skipping 66 matching lines @@
       "python",
       "-u",
       "[BUILD]/scripts/slave/extract_build.py",
       "--target",
       "Release",
       "--build-url",
       "gs://chromium-webrtc/android_rel",
       "--build_revision",
       "12345",
       "--factory-properties",
-      "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"Android Tests (KK Nexus5)\", \"mastername\": \"client.webrtc\", \"parent_buildername\": \"Android Builder\", \"parent_got_revision\": \"12345\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"slavename\": \"slavename\"}",
+      "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"Android Tests (KK Nexus5)\", \"buildnumber\": 1337, \"mastername\": \"client.webrtc\", \"parent_buildername\": \"Android Builder\", \"parent_got_revision\": \"12345\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"slavename\": \"slavename\"}",
       "--build-properties",
-      "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"Android Tests (KK Nexus5)\", \"mastername\": \"client.webrtc\", \"parent_buildername\": \"Android Builder\", \"parent_got_revision\": \"12345\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"slavename\": \"slavename\"}"
+      "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"Android Tests (KK Nexus5)\", \"buildnumber\": 1337, \"mastername\": \"client.webrtc\", \"parent_buildername\": \"Android Builder\", \"parent_got_revision\": \"12345\", \"recipe\": \"webrtc/standalone\", \"revision\": \"12345\", \"slavename\": \"slavename\"}"
     ],
     "name": "extract build"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil, sys, os\nshutil.rmtree(sys.argv[1], True)\nshutil.rmtree(sys.argv[2], True)\ntry:\n  os.remove(sys.argv[3])\nexcept OSError:\n  pass\nfor base, _dirs, files in os.walk(sys.argv[4]):\n  for f in files:\n    if f.endswith('.pyc'):\n      os.remove(os.path.join(base, f))\n",
       "[SLAVE_BUILD]/src/out/Release/debug_info_dumps",
       "[SLAVE_BUILD]/src/out/Release/test_logs",
@@ skipping 319 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@  os.remove(report)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$final_result",
     "reason": "1 out of 16 aggregated steps failed. Failures: Step('tools_unittests') failed with return_code 1",
     "status_code": 1
   }
 ]