LibWeb: Make CSS::Length::m_calculated_style a RefPtr

Let's now have CSS::Length holding raw pointers to things.

Userland/Libraries/LibWeb/CSS/Length.h

@@ -162,7 +162,7 @@ private:
     Type m_type { Type::Undefined };
     float m_value { 0 };
 
-    CalculatedStyleValue* m_calculated_style { nullptr };
+    RefPtr<CalculatedStyleValue> m_calculated_style;
 };
 
 }