Update shelf spacing in Chrome OS according to the MD specs

ash/system/web_notification/ash_popup_alignment_delegate.cc

Index: ash/system/web_notification/ash_popup_alignment_delegate.cc
diff --git a/ash/system/web_notification/ash_popup_alignment_delegate.cc b/ash/system/web_notification/ash_popup_alignment_delegate.cc
index 95b14b015289c0a35eedea9e9ee35a2ea0fdc780..22475f2175f61b944a55536d1d32d82cabc9999b 100644
--- a/ash/system/web_notification/ash_popup_alignment_delegate.cc
+++ b/ash/system/web_notification/ash_popup_alignment_delegate.cc
@@ -62,7 +62,7 @@ void AshPopupAlignmentDelegate::SetSystemTrayHeight(int height) {
   // should be reduced by the height of shelf's shown height.
   if (shelf_->visibility_state() == SHELF_AUTO_HIDE &&
       shelf_->auto_hide_state() == SHELF_AUTO_HIDE_SHOWN) {
-    system_tray_height_ -= kShelfSize - ShelfLayoutManager::kAutoHideSize;
+    system_tray_height_ -= GetShelfConstant(SHELF_SIZE) - kShelfAutoHideSize;
   }
 
   if (system_tray_height_ > 0)