Make more [TreatNullAs=NullString] DOMString arguments nullable

Fix handling of DOMString? combined with [TreatUndefinedAs=NullString] to
behave the same as [TreatNullAs=NulllString] combined with it, and then
change method arguments with TreatNullAs+TreatUndefinedAs to nullable with
just TreatUndefinedAs.

This does not alter behavior or change generated code.

Review URL: https://codereview.chromium.org/367003002

git-svn-id: svn://svn.chromium.org/blink/trunk@177396 bbb929c8-8fbe-4397-9dbb-9b2b20218538