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Screenshot: http://shortn/_AyCiiMx92w

 - Center cards
 - Adjust margin within cards
 - Adjust card width
 - Remove minimum height requirement from cards

Bug: 1125150
Test: browser_tests --gtest_filter=DiagnosticsApp*
Change-Id: I4c4f31182c7c835f6015b15af0f5a3aab64f489d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530744
Commit-Queue: Michael Checo <michaelcheco@google.com>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827994}