Change arrow pointers to hand pointers on buttons in gallery slideshow

BUG=674052
TEST=manually tested as the issue describes
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2622403003
Cr-Commit-Position: refs/heads/master@{#443186}

ui/file_manager/gallery/css/gallery.css

@@ -1270,6 +1270,7 @@ files-toast {
 .slideshow-toolbar > div {
   background-position: center;
   background-repeat: no-repeat;
+  cursor: pointer;
   height: 48px;
   opacity: 0.8;
   pointer-events: auto;