LibWeb: Impose a sane max cookie size

Drop cookies larger than 4KiB. This value is the RFC's recommendation:
https://tools.ietf.org/html/rfc6265#section-6.1

Base/res/html/misc/cookie.html

@@ -14,6 +14,8 @@ <h3>Invalid cookies (the browser should reject these):</h3>
  <label for=invalid3>The cookie expired in the past</label>
  <br /><input id=invalid4 type=button onclick="setCookie(this.value)" value="cookie7=value7; expires=Mon, 23 Jan 1989 08:10:36 GMT" />
  <label for=invalid4>The cookie expired in the past</label>
+ <br /><input id=invalid5 type=button onclick="setTooLargeCookie()" value="cookie10=[more than 4096 chars]" />
+ <label for=invalid5>The cookie is too large</label>
  <br />
 
  <h3>Unretrievable cookies (the browser should accept these but not display them):</h3>
@@ -31,6 +33,11 @@ <h3>Unretrievable cookies (the browser should accept these but not display them)
  document.getElementById('cookies').innerHTML = document.cookie;
  }
 
+ function setTooLargeCookie() {
+ const cookie = 'name=' + 'x'.repeat(4 << 10);
+ setCookie(cookie);
+ }
+
  document.getElementById('cookies').innerHTML = document.cookie;
  </script>
 </body>

Userland/Libraries/LibWeb/Cookie/ParsedCookie.cpp

@@ -30,6 +30,8 @@
 
 namespace Web::Cookie {
 
+static constexpr size_t s_max_cookie_size = 4096;
+
 static void parse_attributes(ParsedCookie& parsed_cookie, StringView unparsed_attributes);
 static void process_attribute(ParsedCookie& parsed_cookie, StringView attribute_name, StringView attribute_value);
 static void on_expires_attribute(ParsedCookie& parsed_cookie, StringView attribute_value);
@@ -43,6 +45,10 @@ static Optional<Core::DateTime> parse_date_time(StringView date_string);
 Optional<ParsedCookie> parse_cookie(const String& cookie_string)
 {
  // https://tools.ietf.org/html/rfc6265#section-5.2
+
+ if (cookie_string.length() > s_max_cookie_size)
+ return {};
+
  StringView name_value_pair;
  StringView unparsed_attributes;