DevTools: Unify font sizes across Settings pane.

Also make DevTools settings screen look similar to Chromium settings page.

BUG=260578
R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/19345002

git-svn-id: svn://svn.chromium.org/blink/trunk@154281 bbb929c8-8fbe-4397-9dbb-9b2b20218538