[NTP] Show scrollbar on background picker

Bug: 864192
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I363ac2306b0ae6c5d5eb45ac891931426e13d52a
Reviewed-on: https://chromium-review.googlesource.com/1140434Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575756}

chrome/browser/resources/local_ntp/custom_backgrounds.css

@@ -179,7 +179,10 @@ html[dir=rtl] .bg-option-text {
   height: 284px;
   overflow-y: scroll;
   padding-left: 8px;
-  width: 100%;
+}
+
+#bg-sel-tiles::-webkit-scrollbar-track {
+  display: none;
 }
 
 html[dir=rtl] #bg-sel-tiles {
@@ -187,10 +190,6 @@ html[dir=rtl] #bg-sel-tiles {
   padding-right: 8px;
 }
 
-#bg-sel-tiles::-webkit-scrollbar {
-    display: none;
-}
-
 #bg-sel-title-bar {
   font-size: 15px;
   height: 52px;