Merge pull request w3c#377 from autokagami/css-masking-1

[css-masking-1] Align with Web IDL specification

Author: dirkschulze

css-masking-1/Overview.bs

@@ -1370,6 +1370,7 @@ Note: A future version of the SVG specification may override this section.
 The <dfn interface>SVGClipPathElement</dfn> interface corresponds to the <a element>clipPath</a> element.
 
 <pre class=idl>
+[Exposed=Window]
 interface SVGClipPathElement : SVGElement {
   readonly attribute SVGAnimatedEnumeration clipPathUnits;
   readonly attribute SVGAnimatedTransformList transform;
@@ -1390,6 +1391,7 @@ interface SVGClipPathElement : SVGElement {
 The <dfn interface>SVGMaskElement</dfn> interface corresponds to the <a element>mask</a> element.
 
 <pre class=idl>
+[Exposed=Window]
 interface SVGMaskElement : SVGElement {
   readonly attribute SVGAnimatedEnumeration maskUnits;
   readonly attribute SVGAnimatedEnumeration maskContentUnits;