LibWeb: Make things aware of box-sizing

Of course, we don't actually *use* the box-sizing property yet, but the
value is applied and shows up in the computed style.

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -39,6 +39,11 @@ enum class AlignItems {
     Stretch,
 };
 
+enum class BoxSizing {
+    BorderBox,
+    ContentBox,
+};
+
 enum class Clear {
     None,
     Left,