Make it clear a DocumentNameCollection can only contain HTMLElements

Make it clear a DocumentNameCollection can only contain HTMLElements.
Only pass HTMLElements to DocumentNameCollection::elementMatches() as
DocumentNameCollection can only contain HTMLElements.

DocumentNameCollection was being handled in HTMLCollection::elementMatches() as
matching any time of Elements, which was inaccurate. Its handling is now moved
to isMatchingHTMLElement(), with other collections matching only HTMLElements.

DocumentNameCollection::item() also now returns an HTMLElement instead of an
Element.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/492453002

git-svn-id: svn://svn.chromium.org/blink/trunk@180653 bbb929c8-8fbe-4397-9dbb-9b2b20218538