Extend chromium_tests trigger to work across masters.

scripts/slave/recipes/chromium.expected/full_chromium_webrtc_Mac_Builder.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.webrtc",
       "--builder",
       "Mac Builder",
@@ skipping 305 matching lines @@
       "--build-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"build_data_dir\": \"[ROOT]/build_data_dir\", \"buildername\": \"Mac Builder\", \"buildnumber\": 571, \"mastername\": \"chromium.webrtc\", \"parent_buildername\": null, \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "name": "package build"
   },
   {
     "cmd": [],
     "name": "trigger",
     "trigger_specs": [
       {
+        "bucket": "master.chromium.webrtc",
         "builder_name": "Mac Tester",
         "properties": {
           "parent_got_angle_revision": "fac9503c46405f77757b9a728eb85b8d7bc6080c",
           "parent_got_angle_revision_cp": "refs/heads/master@{#297276}",
           "parent_got_libjingle_revision": "8e938c8836c7ed23d7fbcddf1dcc2abcb13667d7",
           "parent_got_libjingle_revision_cp": "refs/heads/master@{#134440}",
           "parent_got_libvpx_revision": "9240a98dab275a8a92a3631502866a58dceb2a49",
           "parent_got_libvpx_revision_cp": "refs/heads/master@{#11245}",
           "parent_got_nacl_revision": "d998e125e8253980d76e269b7982aeeefc1b9b50",
           "parent_got_nacl_revision_cp": "refs/heads/master@{#274981}",
@@ skipping 11 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@Mac Tester@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]