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Description:
 remove "hr" tags from diagnostics_app and add
 styling to page header

Screenshots:
 - Diagnostics Page (before) http://shortn/_YdImFO55AY
 - Diagnostics Page (after) http://shortn/_k8NLMVtA6A

Bug: 1125150
Test: browser_tests --gtest_filter=DiagnosticsApp*
Change-Id: I7dcf4d769f758ad79218f3487551fba2f02a1686
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2425384
Commit-Queue: Michael Checo <michaelcheco@google.com>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809998}