chromeos: make mash use MultiUserWindowManagerChromeOS

Ash's MultiUserWindowManagerBridge now creates ash::MultiUserWindowManager
in multi-process mode.

BUG=875111
TEST=covered by tests

Change-Id: I9702b18b453ae4cdde5aad4a8774e2c510e4e9f1
Reviewed-on: https://chromium-review.googlesource.com/c/1355484
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612472}
6 files changed