Files app: Fix grid view scrolling back to top

Scroll issue introduced CL:1636665, which replaced a call to
mergeItems() method in a base class to super, however the immediate
parent class |Grid| also has the mergeItems() method, which was
interfering with the scroll. This CL reverts back to call this method
from |List| base class.

Test: manually
Bug: 993068
Change-Id: I19d225dcc4277a409e824c7cac580c571ba1f051
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1783690
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693052}

ui/file_manager/file_manager/foreground/js/ui/file_grid.js

@@ -237,7 +237,7 @@ class FileGrid extends cr.ui.Grid {
    * @override
    */
   mergeItems(beginIndex, endIndex) {
-    super.mergeItems(beginIndex, endIndex);
+    cr.ui.List.prototype.mergeItems.call(this, beginIndex, endIndex);
 
     const afterFiller = this.afterFiller_;
     const columns = this.columns;