mash: Move SystemTrayDelegate ownership 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 2161121c8fda5a8f5b2b6ad92e22b747eed071e3..5c8d5796e0184fa2a7e35a946287dcba62cbd5d4 100644
--- a/ash/common/wm_shell.cc
+++ b/ash/common/wm_shell.cc
@@ -4,6 +4,8 @@
 
 #include "ash/common/wm_shell.h"
 
+#include "ash/common/system/tray/system_tray_delegate.h"
+
 namespace ash {
 
 // static
@@ -19,4 +21,14 @@ WmShell* WmShell::Get() {
   return instance_;
 }
 
+WmShell::WmShell() {}
+
+WmShell::~WmShell() {}
+
+void WmShell::SetSystemTrayDelegate(
+    std::unique_ptr<SystemTrayDelegate> delegate) {
+  // TODO(jamescook): Create via ShellDelegate once it moves to //ash/common.
+  system_tray_delegate_ = std::move(delegate);
+}
+
 }  // namespace ash