mash: Migrate ShellDelegate ownership and access to WmShell.

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 0ca14db4c4edbcaddb4c017a4c0ae59b114530bb..6c42ca318b2511a8f6fbb49a1166f872ad9a3647 100644
--- a/ash/mus/bridge/wm_shell_mus.cc
+++ b/ash/mus/bridge/wm_shell_mus.cc
@@ -92,8 +92,10 @@ class SessionStateDelegateStub : public SessionStateDelegate {
 
 }  // namespace
 
-WmShellMus::WmShellMus(::ui::WindowTreeClient* client)
-    : client_(client), session_state_delegate_(new SessionStateDelegateStub) {
+WmShellMus::WmShellMus(ShellDelegate* delegate, ::ui::WindowTreeClient* client)
+    : WmShell(delegate),
+      client_(client),
+      session_state_delegate_(new SessionStateDelegateStub) {
   client_->AddObserver(this);
   WmShell::Set(this);