Reland "cros: Add a double separator above context menu notifications"

This reverts commit f69edefd.

Reason for revert: Bug is fixed by adding VIEWS_EXPORT to MenuSeparator.

https://chromium-review.googlesource.com/c/chromium/src/+/1129999

Original change's description:
> Revert "cros: Add a double separator above context menu notifications"
> 
> This reverts commit 1c822163.
> 
> Reason for revert: 
> 
> Linker failures:
> obj/ash/app_menu/app_menu/notification_menu_view.o:notification_menu_view.cc:function ash::NotificationMenuView::NotificationMenuView(ash::NotificationItemView::Delegate*, message_center::SlideOutController::Delegate*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&): error: undefined reference to 'vtable for views::MenuSeparator'
> ../../third_party/binutils/Linux_x64/Release/bin/ld.gold: the vtable symbol may be undefined because the class is missing its key function
> 
> Bug: 861885
> 
> Original change's description:
> > cros: Add a double separator above context menu notifications
> > 
> > Add and remove a double separator with NOTIFICATION_CONTAINER.
> > 
> > Bug: 843761
> > Change-Id: I45fb095079aaaf108a9a18958ab4e431072e588d
> > Reviewed-on: https://chromium-review.googlesource.com/1121243
> > Commit-Queue: Alex Newcomer <newcomer@chromium.org>
> > Reviewed-by: Michael Wasserman <msw@chromium.org>
> > Reviewed-by: James Cook <jamescook@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#573418}
> 
> TBR=jamescook@chromium.org,msw@chromium.org,newcomer@chromium.org
> 
> Change-Id: I10ce18a75e80116fa8d7a530d18e979a2dfbb395
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 843761
> Reviewed-on: https://chromium-review.googlesource.com/1129661
> Reviewed-by: Alex Newcomer <newcomer@chromium.org>
> Commit-Queue: Alex Newcomer <newcomer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#573429}

TBR=jamescook@chromium.org,msw@chromium.org,newcomer@chromium.org

Change-Id: I70f5742583a5531d43ea5ff5b6c2815e691506da
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 861885, 843761
Reviewed-on: https://chromium-review.googlesource.com/1130140Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573480}

ash/app_menu/notification_menu_view.cc

@@ -7,10 +7,12 @@
 #include "ash/app_menu/notification_item_view.h"
 #include "ash/app_menu/notification_menu_header_view.h"
 #include "ash/public/cpp/app_menu_constants.h"
+#include "ui/base/models/menu_separator_types.h"
 #include "ui/gfx/geometry/size.h"
 #include "ui/message_center/public/cpp/notification.h"
 #include "ui/message_center/views/proportional_image_view.h"
 #include "ui/views/controls/menu/menu_item_view.h"
+#include "ui/views/controls/menu/menu_separator.h"
 #include "ui/views/layout/box_layout.h"
 
 namespace ash {
@@ -31,6 +33,10 @@ NotificationMenuView::NotificationMenuView(
   SetLayoutManager(std::make_unique<views::BoxLayout>(
       views::BoxLayout::Orientation::kVertical));
 
+  double_separator_ =
+      new views::MenuSeparator(ui::MenuSeparatorType::DOUBLE_SEPARATOR);
+  AddChildView(double_separator_);
+
   header_view_ = new NotificationMenuHeaderView();
   AddChildView(header_view_);
 }
@@ -42,9 +48,10 @@ bool NotificationMenuView::IsEmpty() const {
 }
 
 gfx::Size NotificationMenuView::CalculatePreferredSize() const {
-  return gfx::Size(
-      views::MenuConfig::instance().touchable_menu_width,
-      header_view_->GetPreferredSize().height() + kNotificationItemViewHeight);
+  return gfx::Size(views::MenuConfig::instance().touchable_menu_width,
+                   double_separator_->GetPreferredSize().height() +
+                       header_view_->GetPreferredSize().height() +
+                       kNotificationItemViewHeight);
 }
 
 void NotificationMenuView::AddNotificationItemView(

ash/app_menu/notification_menu_view.h

@@ -17,6 +17,10 @@ namespace message_center {
 class Notification;
 }
 
+namespace views {
+class MenuSeparator;
+}
+
 namespace ash {
 
 class NotificationMenuHeaderView;
@@ -71,9 +75,13 @@ class APP_MENU_EXPORT NotificationMenuView : public views::View {
   // which is shown.
   std::deque<std::unique_ptr<NotificationItemView>> notification_item_views_;
 
-  // Holds the header and counter texts. Owned by this.
+  // Holds the header and counter texts. Owned by views hierarchy.
   NotificationMenuHeaderView* header_view_ = nullptr;
 
+  // A double separator used to distinguish notifications from context menu
+  // options. Owned by views hierarchy.
+  views::MenuSeparator* double_separator_;
+
   DISALLOW_COPY_AND_ASSIGN(NotificationMenuView);
 };