Make HTMLAllCollection named property getter behave more according to spec

Make HTMLAllCollection named property getter behave more according to spec:
http://www.whatwg.org/specs/web-apps/current-work/multipage/common-dom-interfaces.html#htmlallcollection-0

The following changes were made:
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#htmlallcollection-0
   This change is also consistent with IE11. Mozilla does not seem to
   support enumerating document.all?
3. The named property getter returns more types of elements by name (a, area,
   frame, frameset, iframe). Its still returns select and input elements as
   well, even though this is against spec, for backward compatibility.
   This behavior is consistent with IE11 and Firefox 27.

This CL also gets rid of the custom bindings for namedItem() now that the
bindings generator has better support for returning union types.

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

Review URL: https://codereview.chromium.org/158563004

git-svn-id: svn://svn.chromium.org/blink/trunk@167126 bbb929c8-8fbe-4397-9dbb-9b2b20218538