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- Enable marketing opt-in screen for clamshell mode. Previously
  disabled.
- Change command line switch to feature.
- Enable the marketing screen by default
- Update the styles on the marketing opt-in screen.
- Prepare the screen to be attached to the new ChromebookEmail
  component that will be added.
- Fix broken tests.

Fixed: 1023370
Change-Id: Iaa4f7b595452afafd5d501ca28991b06ea204b1e
Bug: 1023370
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2127926
Commit-Queue: Renato Silva <rrsilva@google.com>
Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755883}