Speculative fix for crash in paged view structure

The crash may occur when dragging an item for reparant in root level
while we need to add a page break item between two items in root level.
The two items happened to have the same position which triggers a
position fix causing a LoadFromMetadata() call during SaveToMetadata().

Changes:
Remove observer when add page break item.

Bug: 900427
Change-Id: Ie416eeb81de47cd531a6c25a16761ce1d141847f
Reviewed-on: https://chromium-review.googlesource.com/c/1313054Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604970}

ash/app_list/paged_view_structure.cc

@@ -94,9 +94,14 @@ void PagedViewStructure::SaveToMetadata() {
 
     if (item_index < item_list->item_count() &&
         !item_list->item_at(item_index)->is_page_break()) {
+      // Remove AppListItemListObserver temporarily to avoid |pages_| being
+      // reloaded.
+      item_list->RemoveObserver(apps_grid_view_);
+
       // There's no "page break" item at the end of current page, so add one to
       // push overflowing items to next page.
       model->AddPageBreakItemAfter(item_list->item_at(item_index - 1));
+      item_list->AddObserver(apps_grid_view_);
     }
   }