chromeos: copies file to ash

This is a copy of c/b/ui/ash/multi_user_window_manager_chromeos* to ash.
I'm doing this as a precursor to a refactoring so you can better see what
changes. I've only changed things necessary to avoid presubmit checks. This
file is purposely not compiled now as it needs to be cleaned up first.

BUG=756085
TEST=nothing new is compiled, so no tests.

Change-Id: I946d9e3b582142761e6519bf754733649d1a7612
Reviewed-on: https://chromium-review.googlesource.com/c/1330078
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607737}

ash/multi_user/OWNERS

@@ -0,0 +1,2 @@
+skuhne@chromium.org
+