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This moves the preview pane into a separate element in
preparation for sharing UI between Settings and login/oobe.

Bug: 730031
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I7dae1a01579e89ca8f4b8fdc3be7393b2310c869
Reviewed-on: https://chromium-review.googlesource.com/557462Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485434}