Consolidate some more ash tray constants.

ash/common/system/tray/system_tray.cc

Index: ash/common/system/tray/system_tray.cc
diff --git a/ash/common/system/tray/system_tray.cc b/ash/common/system/tray/system_tray.cc
index ba1381431294b4db073e463c5f3f3ecf8a1a42e2..27fd181238e3b7f2b90729dc4a58f15fb0cb67ea 100644
--- a/ash/common/system/tray/system_tray.cc
+++ b/ash/common/system/tray/system_tray.cc
@@ -98,8 +98,7 @@ class PaddingTrayItem : public SystemTrayItem {
     gfx::Size GetPreferredSize() const override {
       // The other tray items already have some padding baked in so we have to
       // subtract that off.
-      const int side =
-          kTrayEdgePadding - GetTrayConstant(TRAY_IMAGE_ITEM_PADDING);
+      const int side = kTrayEdgePadding - kTrayImageItemPadding;
       return gfx::Size(side, side);
     }