LibWeb: Mark CalculatedStyleValue::resolve_integer() as const

Userland/Libraries/LibWeb/CSS/StyleValues/CalculatedStyleValue.cpp

@@ -2392,7 +2392,7 @@ Optional<double> CalculatedStyleValue::resolve_number() const
     return {};
 }
 
-Optional<i64> CalculatedStyleValue::resolve_integer()
+Optional<i64> CalculatedStyleValue::resolve_integer() const
 {
     auto result = m_calculation->resolve({}, {});
     if (result.value().has<Number>())

Userland/Libraries/LibWeb/CSS/StyleValues/CalculatedStyleValue.h

@@ -102,7 +102,7 @@ public:
     bool resolves_to_number() const { return m_resolved_type.matches_number(); }
     bool resolves_to_number_percentage() const { return m_resolved_type.matches_number_percentage(); }
     Optional<double> resolve_number() const;
-    Optional<i64> resolve_integer();
+    Optional<i64> resolve_integer() const;
 
     bool contains_percentage() const;