Add ChannelHandle::mojo_handle to mus' ChannelHandle mojom.

services/ui/public/interfaces/channel_handle.mojom

Index: services/ui/public/interfaces/channel_handle.mojom
diff --git a/services/ui/public/interfaces/channel_handle.mojom b/services/ui/public/interfaces/channel_handle.mojom
index 5a4136ec599d8738ad2094fce29617d2b7257aad..06786d56aa30d5d235160f5fb1cf6709111ef4dc 100644
--- a/services/ui/public/interfaces/channel_handle.mojom
+++ b/services/ui/public/interfaces/channel_handle.mojom
@@ -10,4 +10,7 @@ struct ChannelHandle {
 
   // On POSIX, it is one endpoint of a socket pair. On Windows, it is not used.
   handle socket;
+
+  // Message pipe handle used for ChannelMojo.
+  handle<message_pipe>? mojo_handle;
 };