Add use counters for using Attrs as Nodes.

DOM4 makes Attr no longer be a subtype of Node. This adds a use
counter for passing Attr where Node is expected, or calling a method
declared on Node with an Attr.

BUG=305105

Review URL: https://codereview.chromium.org/127723002

git-svn-id: svn://svn.chromium.org/blink/trunk@165262 bbb929c8-8fbe-4397-9dbb-9b2b20218538