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Add interface FilesAppEntry which is the base interface that moving
forward the app UI will converge as base type that can be displayed on
different UI components such as: navigation tree, file list and
breadcrumbs, eventually superseding Entry type.

Add VolumeEntry which implements interface FilesAppEntry to represent a
Volume, this will allow to display Volumes on file list/Right Hand Side
(RHS).

Add EntryList which implements interface FilesAppEntry to represent a
list of entries.  This will be used to implement "My Files" which will
contain a list of VolumeEntry for the volumes: Downloads, Linux Files
(Crostini) and Play Files (ARC++).

Design doc: https://docs.google.com/document/d/1X5XSLKJd0yerL-qFhpb2z9ibUVb_W3gG_tfIV7T_Qt0

Bug: 846587, 835203
Test: Unit-test for the new types.
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Ia2fce233338f8b8e0969b77daf4c77139852c441
Reviewed-on: https://chromium-review.googlesource.com/1086680
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Sasha Morrissey <sashab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572496}