Enable InputDeviceServer/InputDeviceClient in mash.

components/mus/mus_app.cc

Index: components/mus/mus_app.cc
diff --git a/components/mus/mus_app.cc b/components/mus/mus_app.cc
index 75f219276ece862d839c2dde826b59938e48b268..bff03c78cfea87969b1b433354e0a8005c62bb76 100644
--- a/components/mus/mus_app.cc
+++ b/components/mus/mus_app.cc
@@ -180,6 +180,11 @@ void MusApp::Initialize(shell::Connector* connector,
   event_source_ = ui::PlatformEventSource::CreateDefault();
 #endif
 
+  // This needs to happen after DeviceDataManager has been constructed. That
+  // happens either during OzonePlatform or PlatformEventSource initialization,
+  // so keep this line below both of those.
+  input_device_server_.RegisterAsObserver();
+
   if (use_chrome_gpu_command_buffer_) {
     GpuServiceMus::GetInstance();
   } else {
@@ -210,6 +215,8 @@ bool MusApp::AcceptConnection(Connection* connection) {
     connection->AddInterface<Gpu>(this);
   }
 
+  input_device_server_.RegisterInterface(connection);
+
   return true;
 }