New shelf: Fix notification padding from bottom.

According to the spec, new shelf should have 8dip padding between the
top of the shelf and the bottom of the popup notifications.

TEST=manual
BUG=883640

Change-Id: I6186379d221624b76984af69b922f85e36556401
Reviewed-on: https://chromium-review.googlesource.com/1250344Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595468}

ash/system/message_center/ash_popup_alignment_delegate.cc

@@ -10,6 +10,7 @@
 #include "ash/shelf/shelf.h"
 #include "ash/shelf/shelf_constants.h"
 #include "ash/shell.h"
+#include "ash/system/tray/tray_constants.h"
 #include "base/i18n/rtl.h"
 #include "ui/display/display.h"
 #include "ui/display/screen.h"
@@ -24,11 +25,6 @@ namespace {
 
 const int kToastMarginX = 7;
 
-// If there should be no margin for the first item, this value needs to be
-// subtracted to flush the message to the shelf (the width of the border +
-// shadow).
-const int kNoToastMarginBorderAndShadowOffset = 2;
-
 }  // namespace
 
 AshPopupAlignmentDelegate::AshPopupAlignmentDelegate(Shelf* shelf)
@@ -88,7 +84,7 @@ int AshPopupAlignmentDelegate::GetToastOriginX(
 }
 
 int AshPopupAlignmentDelegate::GetBaseline() const {
-  return work_area_.bottom() - kNoToastMarginBorderAndShadowOffset -
+  return work_area_.bottom() - kUnifiedMenuVerticalPadding -
          tray_bubble_height_;
 }