FilesApp: add TODO clean-up comment about {volume,root}-type-icon

The CSS switches on attribute {volume,root}-type-icon, but in practice
there is no real system need for the distinction.

Bug: 992819
Change-Id: Ieca58e2ce1e4bc211c2aea342b73e9f8f467549a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1859415Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705509}

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

@@ -2,6 +2,10 @@
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
+/* TODO(crbug.com/992819): unify root-type-icon and volume-type-icon into one
+ type, called file-type-icon or entry_type-icon say, since there is no real
+ need for the root/volume icon distinction. */
+
 /* Small icons for file types, used in lists and menus. */
 [file-type-icon] {
   background-image: -webkit-image-set(