Test focus behavior for off-screen SVGAElements

This CL adds a test that was missing in
https://codereview.chromium.org/138793005/. The test wasn't included in the
original CL because another CL had to land later in order to make the change
easily testable.

R=pdr@chromium.org

Review URL: https://codereview.chromium.org/156093003

git-svn-id: svn://svn.chromium.org/blink/trunk@166558 bbb929c8-8fbe-4397-9dbb-9b2b20218538