Make it obvious an HTMLFormControlsCollection can only contain HTMLElements

Make it obvious an HTMLFormControlsCollection can only contain HTMLElements
by using tighter typing.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/488703002

git-svn-id: svn://svn.chromium.org/blink/trunk@180680 bbb929c8-8fbe-4397-9dbb-9b2b20218538