Canvas focus outline should match style in html.css.

The default style of an element isn't quite right. The style that
most elements get by default when focused is defined in html.css.
There's no easy way to access this, so this change hardcodes
the values and adds a comment.

BUG=261998

Review URL: https://chromiumcodereview.appspot.com/21643002

git-svn-id: svn://svn.chromium.org/blink/trunk@155532 bbb929c8-8fbe-4397-9dbb-9b2b20218538