bindings: Support sequence<sequence<T>> in method arguments

modules/serviceworkers/Headers.idl depends on generic Dictionary
because the IDL compiler doesn't support sequence<sequence<T>>.
Support it to remove this dependency.

Review URL: https://codereview.chromium.org/597053002

git-svn-id: svn://svn.chromium.org/blink/trunk@182573 bbb929c8-8fbe-4397-9dbb-9b2b20218538