Correct the null focusedFrame() check in Document

Blink revision 170347 introduced a crash because Document::focusedFrame()
fails to check properly for having a null Frame. This corrects the
check.

BUG=358172
R=eseidel@chromium.org, abarth@chromium.org

Review URL: https://codereview.chromium.org/219453004

git-svn-id: svn://svn.chromium.org/blink/trunk@170478 bbb929c8-8fbe-4397-9dbb-9b2b20218538