LibWeb/CSS: Return StringStyleValue's FlyString by reference

Most of the time this copy is completely unnecessary, so let's avoid it!

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

@@ -19,7 +19,7 @@ public:
     }
     virtual ~StringStyleValue() override = default;
 
-    FlyString string_value() const { return m_string; }
+    FlyString const& string_value() const { return m_string; }
     String to_string() const override { return serialize_a_string(m_string); }
 
     bool properties_equal(StringStyleValue const& other) const { return m_string == other.m_string; }