Use more direct Element creation methods

In both these cases we know the kind of Element we want to create so be more specific and avoid some unneeded instructions.

Review URL: https://codereview.chromium.org/161753003

git-svn-id: svn://svn.chromium.org/blink/trunk@167104 bbb929c8-8fbe-4397-9dbb-9b2b20218538