chromeos: exposes functions in ash to get window by window service ids

This is useful in single-process-mash to correlate windows.

BUG=756085
TEST=none

Change-Id: I8845b62754ba5c34d352b49491078e69a35719aa
Reviewed-on: https://chromium-review.googlesource.com/c/1323705
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606130}
5 files changed