Update shelf spacing in Chrome OS according to the MD specs

ash/system/web_notification/web_notification_tray.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/system/web_notification/web_notification_tray.h"
 
 #include "ash/ash_switches.h"
 #include "ash/root_window_controller.h"
 #include "ash/shelf/shelf_layout_manager.h"
 #include "ash/shelf/shelf_layout_manager_observer.h"
@@ skipping 234 matching lines @@
           message_center_tray_.get(),
           true);
 
   // Assume the status area and bubble bottoms are aligned when vertical.
   aura::Window* status_area_window = status_area_widget()->GetNativeView();
   const int max_height =
       wm::IsHorizontalAlignment(GetShelfLayoutManager()->GetAlignment())
           ? GetShelfLayoutManager()->GetIdealBounds().y()
           : status_area_window->GetBoundsInRootWindow().bottom();
 
-  message_center_bubble->SetMaxHeight(std::max(0, max_height - kTraySpacing));
+  message_center_bubble->SetMaxHeight(
+      std::max(0, max_height - GetTrayConstant(TRAY_SPACING)));
   if (show_settings)
     message_center_bubble->SetSettingsVisible();
   message_center_bubble_.reset(
       new WebNotificationBubbleWrapper(this, message_center_bubble));
 
   status_area_widget()->SetHideSystemNotifications(true);
   GetShelfLayoutManager()->UpdateAutoHideState();
   button_->SetBubbleVisible(true);
   SetDrawBackgroundAsActive(true);
   return true;
@@ skipping 237 matching lines @@
 
 message_center::MessageCenterBubble*
 WebNotificationTray::GetMessageCenterBubbleForTest() {
   if (!message_center_bubble())
     return NULL;
   return static_cast<message_center::MessageCenterBubble*>(
       message_center_bubble()->bubble());
 }
 
 }  // namespace ash