LibWeb: Skip some redundant UTF-8 validation in CSS tokenizer

If we're just adding code points to a StringBuilder, there's no need to
revalidate the result.

AK/FlyString.cpp

@@ -31,6 +31,11 @@ ErrorOr<FlyString> FlyString::from_utf8(StringView string)
     return FlyString { TRY(String::from_utf8(string)) };
 }
 
+FlyString FlyString::from_utf8_without_validation(ReadonlyBytes string)
+{
+    return FlyString { String::from_utf8_without_validation(string) };
+}
+
 FlyString::FlyString(String const& string)
 {
     if (string.is_short_string()) {

AK/FlyString.h

@@ -23,6 +23,7 @@ public:
     FlyString() = default;
 
     static ErrorOr<FlyString> from_utf8(StringView);
+    static FlyString from_utf8_without_validation(ReadonlyBytes);
     template<typename T>
     requires(IsOneOf<RemoveCVReference<T>, ByteString, DeprecatedFlyString, FlyString, String>)
     static ErrorOr<String> from_utf8(T&&) = delete;

AK/StringBuilder.cpp

@@ -161,6 +161,11 @@ String StringBuilder::to_string_without_validation() const
     return String::from_utf8_without_validation(string_view().bytes());
 }
 
+FlyString StringBuilder::to_fly_string_without_validation() const
+{
+    return FlyString::from_utf8_without_validation(string_view().bytes());
+}
+
 ErrorOr<FlyString> StringBuilder::to_fly_string() const
 {
     return FlyString::from_utf8(string_view());

AK/StringBuilder.h

@@ -75,6 +75,8 @@ public:
 
     [[nodiscard]] String to_string_without_validation() const;
     ErrorOr<String> to_string() const;
+
+    [[nodiscard]] FlyString to_fly_string_without_validation() const;
     ErrorOr<FlyString> to_fly_string() const;
 
     [[nodiscard]] ErrorOr<ByteBuffer> to_byte_buffer() const;

Userland/Libraries/LibWeb/CSS/Parser/Tokenizer.cpp

@@ -238,7 +238,7 @@ ErrorOr<Vector<Token>> Tokenizer::tokenize(StringView input, StringView encoding
             }
             return {};
         }));
-        return builder.to_string();
+        return builder.to_string_without_validation();
     };
 
     Tokenizer tokenizer { TRY(filter_code_points(input, encoding)) };
@@ -623,7 +623,7 @@ ErrorOr<FlyString> Tokenizer::consume_an_ident_sequence()
         break;
     }
 
-    return result.to_fly_string();
+    return result.to_fly_string_without_validation();
 }
 
 // https://www.w3.org/TR/css-syntax-3/#consume-url-token