Files app: Add background color for selected tree item.

The blue background for a selected tree item should be visible only when the
directory tree is focused.

Bug: 459463
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I8042ab72f9fa83359e107028ca72c9b017829385
Reviewed-on: https://chromium-review.googlesource.com/725207Reviewed-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509739}

ui/file_manager/file_manager/foreground/css/file_manager.css

@@ -207,6 +207,10 @@ div.splitter {
   color: rgb(51, 103, 214);
 }
 
+#directory-tree:focus .tree-row[selected] {
+  background-color: rgb(216, 223, 240);
+}
+
 #directory-tree .tree-row > .root-eject {
   background: -webkit-image-set(
       url(../images/files/ui/eject.png) 1x,