refine window finder for mash

Previously window_finder_chromeos (for mash) looks through the
list of the toplevel browser windows, but it is too simple and
doesn't work well on several cases.

This CL moves the window finding code to ash, and let WindowTree
notify the list of windows at the cursor to the client upon
window moving.

BUG=867074
TEST=ash_unittests

Change-Id: Ib44ad1a927a31c50cc108eee92ddba4d9bd918ee
Reviewed-on: https://chromium-review.googlesource.com/1159346
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582410}
34 files changed