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Define enable_arcore and add some build rules to use it.

Bug: 833511
Change-Id: I716123c2a282d7d123883df81ebbc00a0a883be8
Reviewed-on: https://chromium-review.googlesource.com/1033580
Commit-Queue: Ian Vollick <vollick@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555174}