Use mojo SystemTray interfaces for both mash and classic ash

ash/mus/bridge/wm_shell_mus.cc

Index: ash/mus/bridge/wm_shell_mus.cc
diff --git a/ash/mus/bridge/wm_shell_mus.cc b/ash/mus/bridge/wm_shell_mus.cc
index cc81f8f454d5d62c293274a7d098dc7edca3d8a0..8c948a1aa5f9f31622b5f15e60b0f3eda41c099f 100644
--- a/ash/mus/bridge/wm_shell_mus.cc
+++ b/ash/mus/bridge/wm_shell_mus.cc
@@ -203,6 +203,10 @@ WmRootWindowControllerMus* WmShellMus::GetRootWindowControllerWithDisplayId(
   return nullptr;
 }
 
+bool WmShellMus::IsRunningInMash() const {
+  return true;
+}
+
 WmWindow* WmShellMus::NewWindow(ui::wm::WindowType window_type,
                                 ui::LayerType layer_type) {
   WmWindowMus* window = WmWindowMus::Get(window_tree_client()->NewWindow());