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/Frequency.cpp

@@ -83,4 +83,10 @@ Optional<Frequency::Type> Frequency::unit_from_name(StringView name)
     return {};
 }
 
+NonnullRefPtr<CalculatedStyleValue> Frequency::calculated_style_value() const
+{
+    VERIFY(!m_calculated_style.is_null());
+    return *m_calculated_style;
+}
+
 }