Add mus::mojom::ChannelHandle for passing IPC::ChannelHandle via mojo IPC.

testing/buildbot/chromium.win.json

 {
   "Win 7 Tests x64 (1)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "accessibility_unittests"
       },
       {
@@ skipping 2173 matching lines @@
       {
         "test": "mojo_system_unittests"
       },
       {
         "test": "mojo_view_manager_lib_unittests"
       },
       {
         "test": "mash_unittests"
       },
       {
+        "test": "mus_gpu_unittests"
+      },
+      {
         "test": "mus_ws_unittests"
       },
       {
         "test": "resource_provider_unittests"
       },
       {
         "test": "ui_touch_selection_unittests"
       }
     ]
   },
@@ skipping 10 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "events_unittests"
       }
     ]
   }
 }