Give Ash material design tray items the correct size and layout

ash/system/web_notification/web_notification_tray.cc

Index: ash/system/web_notification/web_notification_tray.cc
diff --git a/ash/system/web_notification/web_notification_tray.cc b/ash/system/web_notification/web_notification_tray.cc
index 5c1d20fe56e5ea427ddf4b927cf91c1a60aa579a..d3e20ba884ce239e67fcefb9972f5a0d92325939 100644
--- a/ash/system/web_notification/web_notification_tray.cc
+++ b/ash/system/web_notification/web_notification_tray.cc
@@ -161,7 +161,8 @@ class WebNotificationButton : public views::CustomButton {
  protected:
   // Overridden from views::ImageButton:
   gfx::Size GetPreferredSize() const override {
-    return gfx::Size(kShelfItemHeight, kShelfItemHeight);
+    const int size = GetTrayConstant(TRAY_ITEM_HEIGHT_LEGACY);
+    return gfx::Size(size, size);
   }
 
   int GetHeightForWidth(int width) const override {