[cssom][css-page] Fixes for properly exposing @page in CSSOM

Hi,

I have fixed bug https://www.w3.org/Bugs/Public/show_bug.cgi?id=22500 and  
would like review of my proposed changes:

https://dvcs.w3.org/hg/csswg/rev/048b4760e33e

commit message:

Expose the margin at-rules in @page as CSSMarginRule, uses type constant  
9. Use a new interface for the declarations in @page and @top-left. Move  
the bulk of CSSStyleDeclaration to a new interface CSSDeclaration, and let  
CSSStyleDeclaration, CSSPageDeclaration and CSSMarginDeclaration inherit  
 from it.

Constant 9 was unused according to wiki.csswg.org/spec/cssom-constants

See http://dev.w3.org/csswg/css-page/ for more context and examples.

cheers
-- 
Simon Pieters
Opera Software