Corrected UA style for a:focus in simpleUserAgentStyleSheet.

simpleUserAgentStyleSheet did not have an outline on a:focus like html.css.
The result was flakiness for the following test depending on whether the full
UA styles had been triggered by a previous test or not:

LayoutTests/fast/events/reveal-link-when-focused.html

R=ojan@chromium.org
BUG=303323

Review URL: https://codereview.chromium.org/25722007

git-svn-id: svn://svn.chromium.org/blink/trunk@158814 bbb929c8-8fbe-4397-9dbb-9b2b20218538