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/common/material_design/material_design_controller.h"
 #include "ash/common/shelf/shelf_constants.h"
 #include "ash/common/shelf/wm_shelf_util.h"
@@ skipping 252 matching lines @@
     max_height = GetShelfLayoutManager()->GetIdealBounds().y();
   } else {
     // Assume the status area and bubble bottoms are aligned when vertical.
     WmWindow* status_area_window =
         WmLookup::Get()->GetWindowForWidget(status_area_widget());
     gfx::Rect bounds_in_root =
         status_area_window->GetRootWindow()->ConvertRectFromScreen(
             status_area_window->GetBoundsInScreen());
     max_height = bounds_in_root.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 240 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