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We currently refer to displays with integer display_ids. This patch
changes them to display::Display for better type safety. It also allows
us to remove some code in display_utils.

Change-Id: Ib996613eebf1b7dcb32e5b683ad33fe6c6a6d084
Reviewed-on: https://chromium-review.googlesource.com/1056932Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558914}