WebKitCSSMatrix compat: define window.WebKitCSSMatrix

Alias WebKitCSSMatrix to DOMMatrix in the same way as SVGMatrix.

Part of whatwg/compat#19

Author: zcorpan

geometry/Overview.bs

@@ -144,7 +144,8 @@ var transformedPoint = point.matrixTransform(matrix);
   <p><var>point</var> creates a new <a interface>DOMPoint</a> object initialized to the same attribute values as <var>point</var>. This new <a interface>DOMPoint</a> is now scaled and the translated by <var>matrix</var>. This resulting <var>transformPoint</var> has the attribute values <code>x: 20</code> and <code>y: 18</code>.</p>
 </div>
 
-For historical reasons, Window objects must also have a writable, configurable, non-enumerable property named <code>SVGPoint</code> whose value is the <a interface>DOMPoint</a> interface object.
+For historical reasons, {{Window}} objects must also have a writable, configurable, non-enumerable
+property named <code>SVGPoint</code> whose value is the <a interface>DOMPoint</a> interface object.
 
 <h2 id='DOMRect'>
 The DOMRect interfaces</h2>
@@ -257,7 +258,9 @@ the <dfn method lt=fromRect() dfn-for=DOMRect id=dom-domrect-fromrect><code>from
   The <dfn>left</dfn> attribute, on getting, must return min(<a for=rectangle>x coordinate</a>, <a for=rectangle>x coordinate</a> + <a for=rectangle>width dimension</a>) of the object it was invoked on.
 </div>
 
-For historical reasons, Window objects must also have a writable, configurable, non-enumerable property named <dfn dfn-type=interface>SVGRect</dfn> whose value is the <a interface>DOMRect</a> interface object.
+For historical reasons, {{Window}} objects must also have a writable, configurable, non-enumerable
+property named <dfn dfn-type=interface>SVGRect</dfn> whose value is the <a interface>DOMRect</a>
+interface object.
 
 <h2 id='DOMQuad'>
 The DOMQuad interface</h2>
@@ -576,7 +579,10 @@ Internal member variables must not be exposed in any way.
 
 The <a interface>DOMMatrix</a> and <a interface>DOMMatrixReadOnly</a> interfaces replace the SVGMatrix interface from SVG [[SVG11]].
 
-For historical reasons, Window objects must also have a writable, configurable, non-enumerable property named <dfn dfn-type=interface>SVGMatrix</dfn> whose value is the <a interface>DOMMatrix</a> interface object.
+For historical reasons, {{Window}} objects must also have writable, configurable, non-enumerable
+properties named <dfn dfn-type=interface>SVGMatrix</dfn> and <dfn
+dfn-type=interface>WebKitCSSMatrix</dfn> whose value is the <a interface>DOMMatrix</a> interface
+object.
 
 <h3 id='dommatrixinit-dictionary'>DOMMatrixInit dictionary</h3>