overview: Fix bug with no recent items showing up in multi display drag.

Use the sessions AddItem instead of grids AddItem. The former updates
the recent items widget and also |num_items_|. This prevents a DCHECK
from crashing when exiting overview, since |num_items_| will have the
wrong value.

Fixed: 1147990
Test: manual
Change-Id: I27f76a6da28572da6fa9f8bd045b239e63d7d19f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533994Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826836}

ash/wm/overview/overview_grid.h

@@ -103,8 +103,8 @@ class ASH_EXPORT OverviewGrid : public SplitViewObserver,
   // of |reposition| and |animate| is false, the stacking order will be adjusted
   // immediately.
   // Note: OverviewSession has versions of the Add/Remove items. Those are
-  // preferred as they update other things like the overview accessibility
-  // annotator.
+  // preferred as they will call into these functions and update other things
+  // like the overview accessibility annotator and the no recent items widget.
   void AddItem(aura::Window* window,
                bool reposition,
                bool animate,

ash/wm/overview/overview_window_drag_controller.cc

@@ -163,17 +163,17 @@ class OverviewItemMoveHelper : public aura::WindowObserver {
     OverviewController* overview_controller =
         Shell::Get()->overview_controller();
     if (overview_controller->InOverviewSession()) {
-      OverviewGrid* target_grid =
-          overview_controller->overview_session()->GetGridWithRootWindow(
-              window->GetRootWindow());
-      // Add |window| to |target_grid| with reposition=false and restack=false,
-      // because soon we will handle both repositioning and restacking anyway.
-      target_grid->AddItemInMruOrder(window, /*reposition=*/false,
+      // OverviewSession::AddItemInMruOrder() will add |window| to the grid
+      // associated with |window|'s root. Do not reposition or restack as we
+      // will soon handle them both anyway.
+      OverviewSession* session = overview_controller->overview_session();
+      session->AddItemInMruOrder(window, /*reposition=*/false,
                                  /*animate=*/false, /*restack=*/false);
-      OverviewItem* item = target_grid->GetOverviewItemContaining(window);
+      OverviewItem* item = session->GetOverviewItemForWindow(window);
+      DCHECK(item);
       item->SetBounds(target_item_bounds_, OVERVIEW_ANIMATION_NONE);
       item->set_should_restack_on_animation_end(true);
-      // The destructor will call |OverviewSession::PositionWindows|.
+      // The destructor will call OverviewSession::PositionWindows().
     }
     delete this;
   }