GitLab

As per crbug.com/788025 and various other discussions:
- Replace "Learn more" link with (?) button
  - Button has a11y text and is Tab stop
- Update text to match crbug.com/721969
- Width = 448px
- At present, NOT removing X (Close) button
- Clean up code

Bug: 788025
Test: Try to play HD content that requires device id / verification.
Verify Harmoniousness of resultant dialog.

Change-Id: Ibcc300a97b6f2edd49d822171467162500d9e859
Reviewed-on: https://chromium-review.googlesource.com/946907Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Greg Levin <glevin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561560}