Gallery: Fix scrolling by touch in the mosaic view.

We cannot scroll the mosaic view by touching since crrev.com/283688.
The touch event is covered by other elements. pointe-events:none does not help
to make it scrollable by touch.

BUG=404960
TEST=test scrolling on link

Review URL: https://codereview.chromium.org/535673004

Cr-Commit-Position: refs/heads/master@{#293281}

ui/file_manager/gallery/css/gallery.css

@@ -1175,6 +1175,7 @@ input[type='checkbox']:checked {
   /* Tile's zoom factor is animated on hover. We apply the transform to
   the entire tile so that the image outline is included into the animation. */
   transition: -webkit-transform 150ms linear;
+  z-index: 50;
 }
 
 /* Mosaic tile's opacity is controlled by |visible| attribute which changes