Improve the UseCounters for HTMLInputElement.webkitdirectory and .incremental

These are already counted in HTMLInputElement::parseAttribute(), but
that code path would not be hit when the IDL attributes are read.

Review URL: https://codereview.chromium.org/297423002

git-svn-id: svn://svn.chromium.org/blink/trunk@174813 bbb929c8-8fbe-4397-9dbb-9b2b20218538