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

testing/buildbot/chromium.linux.json

Index: testing/buildbot/chromium.linux.json
diff --git a/testing/buildbot/chromium.linux.json b/testing/buildbot/chromium.linux.json
index 7c17c1d26b1c65bf3e914dd13e495ec49c00e142..195ddd1dc8de82e7b275c1323b1c574329329210 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -853,6 +853,9 @@
         "test": "mojo_view_manager_lib_unittests"
       },
       {
+        "test": "mus_gpu_unittests"
+      },
+      {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
@@ -1270,6 +1273,9 @@
         "test": "mojo_system_unittests"
       },
       {
+        "test": "mus_gpu_unittests"
+      },
+      {
         "swarming": {
           "can_use_on_swarming_builders": true
         },