mash: Introduce WmSystemTrayNotifier for //ash/common

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 b064ec0b859d452d2553ddca15e90fb8319abfaf..6add8fbb80d1588a3ff1bd6b3d0465cea72a7299 100644
--- a/ash/common/wm_shell.h
+++ b/ash/common/wm_shell.h
@@ -26,6 +26,7 @@ class SystemTrayDelegate;
 class WindowResizer;
 class WmActivationObserver;
 class WmDisplayObserver;
+class WmSystemTrayNotifier;
 class WmWindow;
 
 namespace wm {
@@ -44,6 +45,10 @@ class ASH_EXPORT WmShell {
   static WmShell* Get();
   static bool HasInstance() { return instance_ != nullptr; }
 
+  WmSystemTrayNotifier* system_tray_notifier() {
+    return system_tray_notifier_.get();
+  }
+
   SystemTrayDelegate* system_tray_delegate() {
     return system_tray_delegate_.get();
   }
@@ -132,6 +137,7 @@ class ASH_EXPORT WmShell {
 
   static WmShell* instance_;
 
+  std::unique_ptr<WmSystemTrayNotifier> system_tray_notifier_;
   std::unique_ptr<SystemTrayDelegate> system_tray_delegate_;
 };