Update HTMLFormControlsCollection's named getter to behave according to spec

Update HTMLFormControlsCollection's named getter to behave according to spec:
http://www.whatwg.org/specs/web-apps/current-work/multipage/common-dom-interfaces.html#htmlformcontrolscollection-0

1. namedItem() should be marked as named getter in IDL as per the spec.
   This also means we can get rid of the anonymous named getter in our
   IDL.
2. The named getter should be enumerable as per Web IDL, meaning that
   elements id / names should be enumerated. The supported property names
   are spec'd at:
   http://www.whatwg.org/specs/web-apps/current-work/multipage/common-dom-interfaces.html#htmlformcontrolscollection-0
   Firefox 26 and IE11 both show the names while enumerating. However, IE11
   does not show the ids when enumerating, which is incorrect.
3. The argument to namedItem() named getter should be mandatory. This is
   consistent with the spec, Firefox 26 and IE11.

This CL also gets rid of the custom bindings code needed for the named getter
by improving support for union types in the bindings generator.

R=haraken, arv, tkent
BUG=341269
TEST=fast/dom/htmlformcontrolscollection-enumerated-properties.html

Review URL: https://codereview.chromium.org/157013006

git-svn-id: svn://svn.chromium.org/blink/trunk@166688 bbb929c8-8fbe-4397-9dbb-9b2b20218538