GitLab

Fork Shelf View, Model, Button, TooltipManager, etc.
These provide a baseline of ash/shelf-like functionality.
(show open mash windows, support reordering, tooltips, etc.)

Restructure some ownership patterns (eg. shelf owns model).
Minimal code refactoring, removal and commenting.
(attempt to minimize diff, keep/stub some features)

TODO: Restore missing functionality/tests.

BUG=557406
TEST=mojo:mash_shell's shelf starts looking more like ash.
R=sky@chromium.org,bungeman@google.com

Review URL: https://codereview.chromium.org/1585363002

Cr-Commit-Position: refs/heads/master@{#371866}