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This CL adds Chrome OS Discover UI in separate window.

Window need further changes to match the specs, but this
change mostly adds infrastructure.

Bug: 864686

Change-Id: I2834d8f9efb3660050a31768508992acb1f3f262
Reviewed-on: https://chromium-review.googlesource.com/1182880
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586034}