Stop using shell::GetInstance()->system_tray() in system tray items

ash/system/tray/system_tray_item.cc

Index: ash/system/tray/system_tray_item.cc
diff --git a/ash/system/tray/system_tray_item.cc b/ash/system/tray/system_tray_item.cc
index 44ee5da41eaf646fdd0c13a08667d9d5f06d76ef..868c41f6ff7728199f209def91ef53d748397d3d 100644
--- a/ash/system/tray/system_tray_item.cc
+++ b/ash/system/tray/system_tray_item.cc
@@ -23,7 +23,8 @@ views::View* SystemTrayItem::CreateTrayView(user::LoginStatus status) {
   return NULL;
 }
 
-views::View* SystemTrayItem::CreateDefaultView(user::LoginStatus status) {
+views::View* SystemTrayItem::CreateDefaultView(user::LoginStatus status,
+                                               int bubble_width) {
   return NULL;
 }
 
@@ -83,4 +84,9 @@ bool SystemTrayItem::ShouldShowLauncher() const {
   return true;
 }
 
+bool SystemTrayItem::GuaranteeDefaultViewPreferredSize(
+    user::LoginStatus status) const {
+  return false;
+}
+
 }  // namespace ash