wm: Delete the preview view after removing.

Otherwise destructor which cleans up observers will not be called.

Fixed: 1011330
Change-Id: I62927c958757e8411b3a96e565617cb94547edd1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900139Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712857}

ash/wm/window_mini_view.cc

@@ -61,6 +61,7 @@ void WindowMiniView::SetShowPreview(bool show) {
 
   if (!show) {
     RemoveChildView(preview_view_);
+    delete preview_view_;
     preview_view_ = nullptr;
     return;
   }