Fix NTP background picker when browser is narrow

When the browser is resized to be very narrow the picker should
resize to two columns.

Bug: 850338
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: If73628d85df3bba6860a03949cdfa8b995651264
Reviewed-on: https://chromium-review.googlesource.com/1090997Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565645}

chrome/browser/resources/local_ntp/custom_backgrounds.css

@@ -116,6 +116,12 @@
   z-index: 10000;
 }
 
+@media (max-width: 520px) {
+   #bg-sel-menu {
+     width: 352px;
+   }
+}
+
 #bg-sel-tiles {
   height: 284px;
   overflow-y: scroll;