Update HTMLCollection's named property getter to behave according to spec

Update HTMLCollection's named property getter to behave according to spec:
http://dom.spec.whatwg.org/#htmlcollection

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://dom.spec.whatwg.org/#htmlcollection
   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.

IDL interfaces inheriting HTMLCollection will need similar work (e.g.
HTMLFormControlsCollection). This will be taken care of in follow-up
patches to limit patch size and facilitate review.

R=haraken, arv, tkent
BUG=341269

Review URL: https://codereview.chromium.org/148323008

git-svn-id: svn://svn.chromium.org/blink/trunk@166623 bbb929c8-8fbe-4397-9dbb-9b2b20218538