overview: Fix crash in old ui when dragging item.

Crash was caused when trying to animate layer which only exist in new ui.

Test: manual
Bug: 814917
Change-Id: I95079d32a50c18cd6039e0d451ffee6fc6efc899
Reviewed-on: https://chromium-review.googlesource.com/934965
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539531}

ash/wm/overview/window_selector.cc

@@ -554,9 +554,6 @@ void WindowSelector::InitiateDrag(WindowSelectorItem* item,
   window_drag_controller_.reset(new OverviewWindowDragController(this));
   window_drag_controller_->InitiateDrag(item, location_in_screen);
 
-  if (!IsNewOverviewUi())
-    return;
-
   for (std::unique_ptr<WindowGrid>& grid : grid_list_)
     grid->OnSelectorItemDragStarted(item);
 }
@@ -574,9 +571,6 @@ void WindowSelector::CompleteDrag(WindowSelectorItem* item,
   DCHECK_EQ(item, window_drag_controller_->item());
   window_drag_controller_->CompleteDrag(location_in_screen);
 
-  if (!IsNewOverviewUi())
-    return;
-
   for (std::unique_ptr<WindowGrid>& grid : grid_list_)
     grid->OnSelectorItemDragEnded();
 }

ash/wm/overview/window_selector_item.cc

@@ -515,10 +515,12 @@ class WindowSelectorItem::CaptionContainerView : public views::View {
   gfx::Rect backdrop_bounds() const { return backdrop_bounds_; }
 
   void SetCloseButtonVisibility(bool visible) {
+    DCHECK(close_button_->layer());
     AnimateLayerOpacity(close_button_->layer(), visible);
   }
 
   void SetTitleLabelVisibility(bool visible) {
+    DCHECK(background_->layer());
     AnimateLayerOpacity(background_->layer(), visible);
   }
 
@@ -789,12 +791,16 @@ void WindowSelectorItem::UpdateCannotSnapWarningVisibility() {
 }
 
 void WindowSelectorItem::OnSelectorItemDragStarted(WindowSelectorItem* item) {
+  if (!IsNewOverviewUi())
+    return;
   caption_container_view_->SetCloseButtonVisibility(false);
   if (item == this)
     caption_container_view_->SetTitleLabelVisibility(false);
 }
 
 void WindowSelectorItem::OnSelectorItemDragEnded() {
+  if (!IsNewOverviewUi())
+    return;
   caption_container_view_->SetCloseButtonVisibility(true);
   caption_container_view_->SetTitleLabelVisibility(true);
 }
@@ -937,6 +943,7 @@ void WindowSelectorItem::ActivateDraggedWindow(
 
 void WindowSelectorItem::ResetDraggedWindowGesture() {
   OnSelectorItemDragEnded();
+
   if (tap_down_event_on_title_)
     return;