Deprecate Attr.nodeValue / Attr.textContent

Deprecate Attr.nodeValue / Attr.textContent as those are no longer present
in DOM:
http://dom.spec.whatwg.org/#interface-attr

This behavior is consistent with Firefox (confirmed with version 28), which
also prints a deprecation warning when accessing those 2 properties on an Attr
object. The warning message promotes the use of Attr.value instead.

R=arv@chromium.org, tkent@chromium.org
BUG=305105

Review URL: https://codereview.chromium.org/253843002

git-svn-id: svn://svn.chromium.org/blink/trunk@172803 bbb929c8-8fbe-4397-9dbb-9b2b20218538