Move id attribute to parent Element interface

Move id attribute from SVGElement / HTMLElement to their Element parent
interface to match the latest DOM specification and avoid duplication:
http://dom.spec.whatwg.org/#dom-element-id

This behavior is consistent with Firefox.

There is no web-exposed behavior change because we haven't moved
properties from instances to their prototype, as per the Web IDL
specification (http://dev.w3.org/2006/webapi/WebIDL/#es-attributes).

Review URL: https://chromiumcodereview.appspot.com/21022014

git-svn-id: svn://svn.chromium.org/blink/trunk@155245 bbb929c8-8fbe-4397-9dbb-9b2b20218538