DevTools: Audit results: avoid horizontal scrollbar.

When audit results pane is shinked it's items content is wrapped, but
headers aren't. This cause unwanted horizontal scroll.

This patch adds 'overflow: hidden' rule, so headers will be properly cutted.

BUG=405465

Review URL: https://codereview.chromium.org/493023002

git-svn-id: svn://svn.chromium.org/blink/trunk@180732 bbb929c8-8fbe-4397-9dbb-9b2b20218538