LibWeb: Plumb calculated StyleValues into CSS::Length

This is a bit hackish, but this way the existance of the calc()
becomes transparent to the user who just wants a Length and doesn't
care where it came from.

Userland/Libraries/LibWeb/Forward.h

@@ -14,6 +14,7 @@ enum class Source;
 }
 
 namespace Web::CSS {
+class CalculatedStyleValue;
 class CSSRule;
 class CSSImportRule;
 class CSSStyleDeclaration;