mash: Migrate ShellDelegate ownership and access to WmShell.

ash/common/wm_shell.cc

Index: ash/common/wm_shell.cc
diff --git a/ash/common/wm_shell.cc b/ash/common/wm_shell.cc
index bab48cce697e7237b8977401a9b6de60aae3dc6a..58f1cab1df276271b12509063214079bad90b079 100644
--- a/ash/common/wm_shell.cc
+++ b/ash/common/wm_shell.cc
@@ -53,8 +53,9 @@ void WmShell::RemoveShellObserver(ShellObserver* observer) {
   shell_observers_.RemoveObserver(observer);
 }
 
-WmShell::WmShell()
-    : focus_cycler_(new FocusCycler),
+WmShell::WmShell(ShellDelegate* delegate)
+    : delegate_(delegate),
+      focus_cycler_(new FocusCycler),
       system_tray_notifier_(new SystemTrayNotifier),
       window_selector_controller_(new WindowSelectorController()) {}
 
@@ -84,6 +85,10 @@ void WmShell::SetKeyboardUI(std::unique_ptr<KeyboardUI> keyboard_ui) {
   keyboard_ui_ = std::move(keyboard_ui);
 }
 
+void WmShell::DeleteShellDelegate() {
+  delegate_.reset();
+}
+
 void WmShell::SetMediaDelegate(std::unique_ptr<MediaDelegate> delegate) {
   media_delegate_ = std::move(delegate);
 }