Update shelf visibility in ~TabletModeWindowDragDelegate.

Added UpdateShelfVisibility in ~TabletModeWindowDragDelegate to keep
shelf in correct state after drag a window from top.
Most of the cases shelf can achieve a correct state after drag except for
dragging a browser window from top. The problem is that we delete drag
details in EndingForDraggingWindow for app windows but we can't do the
same for browser window.
Then ShelfLayoutManager::IsDraggingWindowFromTopOrCaptionArea
will drop into wrong condition in this case, which will cause the shelf
into a wrong state at the end of drag.

Tried to UpdateShelfVisibility again in ~TabletModeWindowDragDelegate
when drag details has already been deleted for all the window drags.

Bug: 882962
Change-Id: I6915bcfd7ee1890d40ca5d422c913552a487daf1
Reviewed-on: https://chromium-review.googlesource.com/1219934Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590529}

ash/wm/tablet_mode/tablet_mode_window_drag_delegate.cc

@@ -72,7 +72,9 @@ TabletModeWindowDragDelegate::TabletModeWindowDragDelegate()
       split_view_drag_indicators_(std::make_unique<SplitViewDragIndicators>()),
       weak_ptr_factory_(this) {}
 
-TabletModeWindowDragDelegate::~TabletModeWindowDragDelegate() = default;
+TabletModeWindowDragDelegate::~TabletModeWindowDragDelegate() {
+  Shell::Get()->UpdateShelfVisibility();
+}
 
 void TabletModeWindowDragDelegate::StartWindowDrag(
     aura::Window* dragged_window,
@@ -82,7 +84,7 @@ void TabletModeWindowDragDelegate::StartWindowDrag(
 
   PrepareForDraggedWindow(location_in_screen);
 
-  // Update the shelf's visibility.
+  // Update the shelf's visibility to keep shelf visible during drag.
   RootWindowController::ForWindow(dragged_window_)
       ->GetShelfLayoutManager()
       ->UpdateVisibilityState();