Add WebDocument::focusedElement.

WebDocument::focusedNode should be focusedElement.
DOM, HTML, SVG specifications assume only elements can be focused.
Blink exposes focusedNode api and returns WebNode casting from Element*.
This change would make a cleaner API and expose focused
WebElement which is more rich than WebNode.

BUG=258763

Review URL: https://codereview.chromium.org/173283003

git-svn-id: svn://svn.chromium.org/blink/trunk@167657 bbb929c8-8fbe-4397-9dbb-9b2b20218538