Flip GN bots to be purely driven by //testing/buildbot/*.json files for tests.

scripts/slave/recipes/chromium_gn.expected/full_chromium_webrtc_fyi_Mac_GN.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromium.webrtc.fyi",
       "--builder",
       "Mac GN",
@@ skipping 176 matching lines @@
       "--goma-dir",
       "[BUILD]/goma",
       "--",
       "base_unittests",
       "all"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "compile"
   },
   {
+    "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
-      "[DEPOT_TOOLS]/gn.py",
-      "--root=[SLAVE_BUILD]/src",
-      "check",
-      "//out/Release"
+      "[BUILD]/scripts/slave/runtest.py",
+      "--target",
+      "Release",
+      "--factory-properties",
+      "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"buildername\": \"Mac GN\", \"buildnumber\": 571, \"mastername\": \"chromium.webrtc.fyi\", \"recipe\": \"chromium_gn\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
+      "--build-properties",
+      "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"buildername\": \"Mac GN\", \"buildnumber\": 571, \"mastername\": \"chromium.webrtc.fyi\", \"recipe\": \"chromium_gn\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
+      "--annotate=gtest",
+      "--test-type=base_unittests",
+      "--step-name=base_unittests",
+      "--test-launcher-summary-output",
+      "/path/to/tmp/json",
+      "--generate-json-file",
+      "-o",
+      "gtest-results/base_unittests",
+      "--builder-name=Mac GN",
+      "--slave-name=TestSlavename",
+      "--build-number=571",
+      "base_unittests"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "gn check"
+    "name": "base_unittests"
   }
 ]