Bindings: Remove special cases for DOMStringList

When DOMStringList was added, special cases were also added so that it
could masquerade as sequence<DOMString> and DOMString[] as well, since
those Web IDL types were not supported. Now that Blink IDL understands
sequence<T> and T[] we can remove the special cases.

R=haraken@chromium.org,nbarth@chromium.org
BUG=239771

Review URL: https://codereview.chromium.org/47073007

git-svn-id: svn://svn.chromium.org/blink/trunk@161079 bbb929c8-8fbe-4397-9dbb-9b2b20218538