Fix a crash when apps grid gap is enabled

Background:
A crash occur when a page break item exists in folder. (Not sure why a
page break item gets into folder.)

Changes:
Remove the page break item from folder if it exists.

Bug: 873485
Change-Id: I0cc435ae8d47c056eeec4b04a8afe7f5cc989588
Reviewed-on: https://chromium-review.googlesource.com/1172071
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583125}

chrome/browser/ui/app_list/app_list_syncable_service.cc

@@ -78,6 +78,10 @@ void UpdateSyncItemFromSync(const sync_pb::AppListSpecifics& specifics,
 bool UpdateSyncItemFromAppItem(const ChromeAppListItem* app_item,
                                AppListSyncableService::SyncItem* sync_item) {
   DCHECK_EQ(sync_item->item_id, app_item->id());
+
+  // Page breaker should not be added in a folder.
+  DCHECK(!app_item->is_page_break() || app_item->folder_id().empty());
+
   bool changed = false;
   if (sync_item->parent_id != app_item->folder_id()) {
     sync_item->parent_id = app_item->folder_id();
@@ -1201,11 +1205,22 @@ void AppListSyncableService::PruneRedundantPageBreakItems() {
     }
   }
 
-  // Remove the trailing "page break" itme if it exists.
+  // Remove the trailing "page break" item if it exists.
   if (!top_level_sync_items.empty() &&
       IsPageBreakItem(*top_level_sync_items.back())) {
     DeleteSyncItem(top_level_sync_items.back()->item_id);
   }
+
+  // Remove all the "page break" items that are in folder. No such item should
+  // exist in folder. It should be safe to remove them if it do occur.
+  for (auto iter = sync_items_.begin(); iter != sync_items_.end();) {
+    const auto* sync_item = (iter++)->second.get();
+    if (IsTopLevelAppItem(*sync_item) || !IsPageBreakItem(*sync_item))
+      continue;
+
+    LOG(ERROR) << "Delete a page break item in folder: " << sync_item->item_id;
+    DeleteSyncItem(sync_item->item_id);
+  }
 }
 
 }  // namespace app_list

chrome/browser/ui/app_list/app_list_syncable_service.h

@@ -260,7 +260,7 @@ class AppListSyncableService : public syncer::SyncableService,
   // TODO(http://crbug.com/794724): Remove after M65 goes stable.
   void RemoveDriveAppItems();
 
-  // Returns a list of sync items sorted by item ordinal.
+  // Returns a list of top level sync items sorted by item ordinal.
   std::vector<SyncItem*> GetSortedTopLevelSyncItems() const;
 
   // Remove leading, trailing and duplicate "page break" items in sorted top