Consolidate some more ash tray constants.

ash/common/system/tray/tray_popup_header_button.cc

Index: ash/common/system/tray/tray_popup_header_button.cc
diff --git a/ash/common/system/tray/tray_popup_header_button.cc b/ash/common/system/tray/tray_popup_header_button.cc
index a945311a940a181ceefc866d4a9c81055ed778e4..07334c6cbdcfab93b2a9ffac8d2fd5c2ca2d85a2 100644
--- a/ash/common/system/tray/tray_popup_header_button.cc
+++ b/ash/common/system/tray/tray_popup_header_button.cc
@@ -57,8 +57,7 @@ const char* TrayPopupHeaderButton::GetClassName() const {
 }
 
 gfx::Size TrayPopupHeaderButton::GetPreferredSize() const {
-  int size = GetTrayConstant(TRAY_POPUP_ITEM_MIN_HEIGHT);
-  return gfx::Size(size, size);
+  return gfx::Size(kTrayPopupItemMinHeight, kTrayPopupItemMinHeight);
 }
 
 void TrayPopupHeaderButton::StateChanged(ButtonState old_state) {