TreeScope::focusedNode should be adjustedFocusedElement.

- Only elements can be focusable. TreeScope::focusedNode must return
  Element.
- TreeScope::focusedNode is in conflict with Document::focusedNode. We
  should add something to the function name because we're going to
  rename Document::focusedNode to focusedElement.

BUG=258763
R=hayato@chromium.org

Review URL: https://codereview.chromium.org/18203004

git-svn-id: svn://svn.chromium.org/blink/trunk@154270 bbb929c8-8fbe-4397-9dbb-9b2b20218538