Reland: Web MIDI: split build rules for media/midi

testing/buildbot/chromium_trybot.json

 {
   "compile_targets": [
     "all"
   ],
   "filter_compile_builders": [
     "win_chromium_compile_dbg",
     "win_chromium_rel_swarming",
     "win_chromium_x64_rel_swarming",
     "win8_chromium_rel"
   ],
@@ skipping 195 matching lines @@
         "can_use_on_swarming_builders": true
       },
       "test": "media_unittests"
     },
     {
       "swarming": {
         "can_use_on_swarming_builders": true
       },
       "test": "message_center_unittests"
     },
+    {
+      "swarming": {
+        "can_use_on_swarming_builders": true
+      },
+      "test": "midi_unittests"
+    },
     "mojo_common_unittests",
     "mojo_public_bindings_unittests",
     "mojo_public_environment_unittests",
     "mojo_public_system_unittests",
     "mojo_public_utility_unittests",
     "mojo_system_unittests",
     "nacl_loader_unittests",
     {
       "swarming": {
         "can_use_on_swarming_builders": true
@@ skipping 138 matching lines @@
       ],
       "platforms": [
         "linux"
       ],
       "test": "ozone_unittests"
     }
   ],
   "non_filter_builders": [],
   "non_filter_tests_builders": []
 }