Add another check on Document.elementFromPoint() not to return Document.

As rendererFromPoint() can return a Document's renderer and
elementFromPoint() cannot return the Document, such case it should
return null.

TEST=document-elementFromPoint-empty-document.html
R=tkent,hayato
BUG=313384

Review URL: https://codereview.chromium.org/105523007

git-svn-id: svn://svn.chromium.org/blink/trunk@164003 bbb929c8-8fbe-4397-9dbb-9b2b20218538