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These will be used for ARC++ windows.

This also moves the back button asset to ash/public/cpp/vector_icons
where they should be.

BUG=822960

Change-Id: Ie26b11c1eacbef8cce9e6f07a17ba8daf2c80754
Reviewed-on: https://chromium-review.googlesource.com/966942Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543895}