Allow filenames to be selectable in file:/// directory listings

Regular file names (but not folder names, or metadata) were marked
"draggable", which implcitly made them non-user-selectable.

Explicitly mark them with "user-select: auto".

Bug: 1083760
Change-Id: Ide7a7153c0dcd9331dea33323eeac36599bb7757
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2268298
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Reviewed-by: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783107}

net/base/dir_header.html

@@ -170,6 +170,7 @@ window.addEventListener('DOMContentLoaded', onLoad);
   a.icon {
     -webkit-padding-start: 1.5em;
     text-decoration: none;
+    user-select: auto;
   }
 
   a.icon:hover {