LibWeb: Add referrer policy to Fetch::Infrastructure::Request

The enum is in its own directory and namespace as there's a standalone
spec for it, that will later also house AOs.

- https://w3c.github.io/webappsec-referrer-policy/
- https://www.w3.org/TR/referrer-policy/

Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.h

@@ -257,6 +257,9 @@ public:
     [[nodiscard]] ReferrerType const& referrer() const { return m_referrer; }
     void set_referrer(ReferrerType referrer) { m_referrer = move(referrer); }
 
+    [[nodiscard]] Optional<ReferrerPolicy::ReferrerPolicy> const& referrer_policy() const { return m_referrer_policy; }
+    void set_referrer_policy(Optional<ReferrerPolicy::ReferrerPolicy> referrer_policy) { m_referrer_policy = move(referrer_policy); }
+
     [[nodiscard]] ResponseTainting response_tainting() const { return m_response_tainting; }
     void set_response_tainting(ResponseTainting response_tainting) { m_response_tainting = response_tainting; }
 
@@ -364,7 +367,8 @@ private:
     ReferrerType m_referrer { Referrer::Client };
 
     // https://fetch.spec.whatwg.org/#concept-request-referrer-policy
-    // FIXME: A request has an associated referrer policy, which is a referrer policy. Unless stated otherwise it is the empty string.
+    // A request has an associated referrer policy, which is a referrer policy. Unless stated otherwise it is the empty string.
+    Optional<ReferrerPolicy::ReferrerPolicy> m_referrer_policy;
 
     // https://fetch.spec.whatwg.org/#concept-request-mode
     // A request has an associated mode, which is "same-origin", "cors", "no-cors", "navigate", or "websocket". Unless stated otherwise, it is "no-cors".

Userland/Libraries/LibWeb/Forward.h

@@ -357,6 +357,10 @@ namespace Web::Platform {
 class Timer;
 }
 
+namespace Web::ReferrerPolicy {
+enum class ReferrerPolicy;
+}
+
 namespace Web::RequestIdleCallback {
 class IdleDeadline;
 }

Userland/Libraries/LibWeb/ReferrerPolicy/ReferrerPolicy.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Web::ReferrerPolicy {
+
+// https://w3c.github.io/webappsec-referrer-policy/#enumdef-referrerpolicy
+enum class ReferrerPolicy {
+    NoReferrer,
+    NoReferrerWhenDowngrade,
+    SameOrigin,
+    Origin,
+    StrictOrigin,
+    OriginWhenCrossOrigin,
+    StrictOriginWhenCrossOrigin,
+    UnsafeURL,
+};
+
+}