Make HTMLOptionsCollection's named property getter behave more according to spec

Make HTMLOptionsCollection's named property getter behave more according to spec:
http://www.whatwg.org/specs/web-apps/current-work/multipage/common-dom-interfaces.html#htmloptionscollection

The changes are:
- namedGetter() method should be marked as a named property getter in IDL. As a
  result, we can remove the anonymous named property getter from our IDL.
- namedGetter() argument is now mandatory. This is consistent with Firefox 27
  and other HTML Collections' namedGetter. The argument is optional in IE11
  though.
- 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#htmloptionscollection
  This is consistent with the behavior of Firefox 27. In IE11,
  HTMLSelectElement.options seems to return an HTMLSelectElement instead of
  an HTMLOptionsCollection.

This CL also gets rid of the custom bindings code for the named property getter
as our bindings generator now has better support for returning union types.

The named property getter still does not completely match the specification
though (or the behavior of Firefox 27). It is supposed to always return the
first matching Element. However, in case of duplicates, we currently return a
NodeList type containing all matching Elements. I did not change this behavior
as this was our previous behavior and IE11 also seems to return all matching
Elements in this case.

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

Review URL: https://codereview.chromium.org/147053007

git-svn-id: svn://svn.chromium.org/blink/trunk@166976 bbb929c8-8fbe-4397-9dbb-9b2b20218538