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 - Update css variables to conform to responsiveness specs at
   different breakpoints.

Spec: https://carbon.googleplex.com/cros-ux/pages/scanning/responsiveness

Screenshots:

 600px <= width < 768px: http://shortn/_jbKNxJrAMB
 768px <= width < 960px: http://shortn/_Jy9QYyvf63
 1280px: http://shortn/_ARoXo5FcD4


Bug: 1059779
Test: browser_tests --gtest_filter=ScanningAppBrowserTest.All
Change-Id: I30390bde40f0a2bb033f30e7b12b92aea8669239
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2514660Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Commit-Queue: Michael Checo <michaelcheco@google.com>
Cr-Commit-Position: refs/heads/master@{#824642}