mash: Move SystemTrayDelegate ownership to WmShell

ash/common/wm_shell.h

Index: ash/common/wm_shell.h
diff --git a/ash/common/wm_shell.h b/ash/common/wm_shell.h
index 3a34e364139bc846f97dccab6e4602c15ce4d265..a61fb2d7967d965d7bd0b8de8afd4f3eb87fe486 100644
--- a/ash/common/wm_shell.h
+++ b/ash/common/wm_shell.h
@@ -22,6 +22,7 @@ class AccessibilityDelegate;
 class MruWindowTracker;
 class SessionStateDelegate;
 class ShellObserver;
+class SystemTrayDelegate;
 class WindowResizer;
 class WmActivationObserver;
 class WmDisplayObserver;
@@ -43,6 +44,10 @@ class ASH_EXPORT WmShell {
   static WmShell* Get();
   static bool HasInstance() { return instance_ != nullptr; }
 
+  SystemTrayDelegate* system_tray_delegate() {
+    return system_tray_delegate_.get();
+  }
+
   virtual MruWindowTracker* GetMruWindowTracker() = 0;
 
   // Creates a new window used as a container of other windows. No painting is
@@ -115,10 +120,17 @@ class ASH_EXPORT WmShell {
   virtual void RemoveShellObserver(ShellObserver* observer) = 0;
 
  protected:
-  virtual ~WmShell() {}
+  WmShell();
+  virtual ~WmShell();
+
+  void SetSystemTrayDelegate(std::unique_ptr<SystemTrayDelegate> delegate);
 
  private:
+  friend class Shell;
+
   static WmShell* instance_;
+
+  std::unique_ptr<SystemTrayDelegate> system_tray_delegate_;
 };
 
 }  // namespace ash