[directorytree] Show CSS focus rings only for keyboard interaction

DirectoryTree displays focus rings when the keyboard or pointer device
(mouse/pointer/touch) is used for interaction in files-ng.

Change that: only show focus rings for keyboard interaction, using the
global focus-outline-visible class provided by the cr.ui.focus_outline
visibility manager.

Bug: 1062902, 992819
Change-Id: Idb94a7be5582cfa4ae7429f04d53b95c8c855596
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2114512Reviewed-by: Alex Danilo <adanilo@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753531}

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

@@ -309,7 +309,7 @@ html:not(.col-resize) #directory-tree
   background-color: var(--google-grey-100);
 }
 
-#directory-tree:focus .tree-row[selected] > .file-row {
+html.focus-outline-visible #directory-tree:focus .tree-row[selected] > .file-row {
   border: 1px solid var(--google-blue-600);
 }