Fix accessability for web notification tray.

ash/system/tray/system_tray_bubble.cc

Index: ash/system/tray/system_tray_bubble.cc
diff --git a/ash/system/tray/system_tray_bubble.cc b/ash/system/tray/system_tray_bubble.cc
index 695bb9d0c42da3b47ca6c51e3fd6fa3d69b462d1..7ef24c7a4b8818ca317db71b659b66d032803122 100644
--- a/ash/system/tray/system_tray_bubble.cc
+++ b/ash/system/tray/system_tray_bubble.cc
@@ -333,6 +333,10 @@ void SystemTrayBubble::OnClickedOutsideView() {
     bubble_widget_->Close();
 }
 
+string16 SystemTrayBubble::GetAccessibleName() {
+  return tray_->GetAccessibleName();
+}
+
 void SystemTrayBubble::DestroyItemViews() {
   for (std::vector<ash::SystemTrayItem*>::iterator it = items_.begin();
        it != items_.end();