Remove custom FormData.append() bindings code.

The overload resolution needed for the two FormData.append() methods can
be handled directly by generated code, so drop the custom code we had
in place for this API.

Along with it, handle the conversion of append()'s DOMString arguments
per WebIDL (as it is now), removing the special treatment of null and
undefined arguments. This also aligns with how others (Gecko, Trident)
handle these arguments.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/209933002

git-svn-id: svn://svn.chromium.org/blink/trunk@169841 bbb929c8-8fbe-4397-9dbb-9b2b20218538