[cssom] 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. https://www.w3.org/Bugs/Public/show_bug.cgi?id=22500

Author: Simon Pieters