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

testing/buildbot/chromium.linux.json

 {
   "Android GN": {
     "gtest_tests": [
       {
         "test": "android_webview_unittests"
       },
       {
         "test": "base_unittests"
       },
       {
@@ skipping 835 matching lines @@
       {
         "test": "mojo_surfaces_lib_unittests"
       },
       {
         "test": "mojo_system_unittests"
       },
       {
         "test": "mojo_view_manager_lib_unittests"
       },
       {
+        "test": "mus_gpu_unittests"
+      },
+      {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "mus_ws_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "nacl_loader_unittests"
@@ skipping 397 matching lines @@
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "mojo_public_system_unittests"
       },
       {
         "test": "mojo_system_unittests"
       },
       {
+        "test": "mus_gpu_unittests"
+      },
+      {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "mus_ws_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "nacl_loader_unittests"
@@ skipping 467 matching lines @@
       }
     ],
     "scripts": [
       {
         "name": "nacl_integration",
         "script": "nacl_integration.py"
       }
     ]
   }
 }