DevTools: [Settings] Make the page title/contents separator 100% wide

Drive-by: slightly condense the whitespace around the separator, as it looks too comfortable

R=pfeldman@chromium.org, pfeldman, vsevik
BUG=315058

Review URL: https://codereview.chromium.org/59613003

git-svn-id: svn://svn.chromium.org/blink/trunk@161334 bbb929c8-8fbe-4397-9dbb-9b2b20218538