LibWeb: Ensure PercentageOr<T>::resolved() returns a concrete T

Which is to say, a T where `is_calculated()` is false.

As is becoming a repeating theme with CSS types, we have two states for
a FooPercentage that is a `calc()` expression: Either the FooPercentage
holds the CalculatedStyleValue directly, or it holds a Foo which itself
holds the CalculatedStyleValue. The first case was already handled to
return Foo, and with this patch, the second is too. :^)

Userland/Libraries/LibWeb/CSS/Length.h

@@ -76,6 +76,7 @@ public:
     }
 
     float raw_value() const { return m_value; }
+    NonnullRefPtr<CalculatedStyleValue> calculated_style_value() const;
 
     float to_px(Layout::Node const&) const;