Move HTMLInputElement method argument defaults into IDL file

The stepUp()/stepDown() methods' argument should default to 1, and the
setRangeText() method's selectionMode argument should default to
"preserve".

This matches the specification, and doesn't change behavior since these
defaults were already implemented in the C++ code.

BUG=258153

Review URL: https://codereview.chromium.org/471163006

git-svn-id: svn://svn.chromium.org/blink/trunk@180647 bbb929c8-8fbe-4397-9dbb-9b2b20218538