LibWeb: Support individual scale CSS property

Author: https://github.com/awesomekling
Commit: 9a7c9286c4
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2506
Reviewed-by: https://github.com/shannonbooth

Libraries/LibWeb/Forward.h

@@ -205,6 +205,7 @@ class Resolution;
 class ResolutionOrCalculated;
 class ResolutionStyleValue;
 class RotationStyleValue;
+class ScaleStyleValue;
 class Screen;
 class ScreenOrientation;
 class ScrollbarGutterStyleValue;