LibWeb: Use rect value in CSS clip property

When a rect value is passed to the clip property via CSS, keep it in
ComputedValues so that at a later stage can make use of it.

Userland/Libraries/LibWeb/Forward.h

@@ -32,6 +32,7 @@ class BackgroundStyleValue;
 class BorderRadiusStyleValue;
 class BorderRadiusShorthandStyleValue;
 class BorderStyleValue;
+class Clip;
 class CalculatedStyleValue;
 class ColorStyleValue;
 class ContentStyleValue;