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 - Add custom css vars that can be updated at different
   breakpoints to conform to the spec.
 - Align scan preview and dropdown section.
 - Update html markup to support panel layout.

Screenshot: http://shortn/_vmZWXoAuiQ

Bug: 1059779
Test: browser_tests --gtest_filter=ScanningUIBrowserTest.All
Change-Id: I36927a2c2b9e7370916aa8d1010083bf2f6d601e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2504209
Commit-Queue: Michael Checo <michaelcheco@google.com>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821782}