Use the maximal outline size as a slop when avoiding content clips.

This is how outlines are accounted for in repaint rects; while an
unfortunate hack, this is how we deal with this situation elsewhere,
and handling it better would likely involve fixing the "maximal outline
size" code everywhere.

This change fixes a bug in which focus rings leaked outside the clip.

BUG=271295

Review URL: https://chromiumcodereview.appspot.com/23203004

git-svn-id: svn://svn.chromium.org/blink/trunk@156160 bbb929c8-8fbe-4397-9dbb-9b2b20218538