[CSSOM] CSSRule.NAMESPACE_RULE

Two comments about this:

1. this is not at all implemented by browsers so editing a stylesheet
    with namespace rules in an editing tool is currently impossible.
    I urge browser vendors to implement and ship this as soon as
    possible...

2. CSSRule.NAMESPACE_RULE cannot be 7 [1] because that value is
    already taken by SVG [2] for SVGCSSRule.COLOR_PROFILE_RULE.
    (please note Tantek already mentioned it here in www-style
    exactly seven years ago, eh...).

[1] http://dev.w3.org/csswg/cssom/#the-cssrule-interface
[2] http://www.w3.org/TR/SVG11/types.html#InterfaceSVGCSSRule

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