Document::focusedNode should be focusedElement, which returns Element*.

This is a preparation to remove focus-related features from Node class.
This was inspired by akling's WebKit changes.

BUG=258763

Review URL: https://chromiumcodereview.appspot.com/19504003

git-svn-id: svn://svn.chromium.org/blink/trunk@154446 bbb929c8-8fbe-4397-9dbb-9b2b20218538