LibWeb: Add naive support for the Document.referrer attribute

It always returns the empty string for now.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -964,4 +964,10 @@ bool Document::has_a_style_sheet_that_is_blocking_scripts() const
     return browsing_context()->container_document()->m_script_blocking_style_sheet_counter > 0;
 }
 
+String Document::referrer() const
+{
+    // FIXME: Return the document's actual referrer.
+    return "";
+}
+
 }

Userland/Libraries/LibWeb/DOM/Document.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -58,6 +58,8 @@ public:
     String cookie(Cookie::Source = Cookie::Source::NonHttp);
     void set_cookie(String, Cookie::Source = Cookie::Source::NonHttp);
 
+    String referrer() const;
+
     bool should_invalidate_styles_on_attribute_changes() const { return m_should_invalidate_styles_on_attribute_changes; }
     void set_should_invalidate_styles_on_attribute_changes(bool b) { m_should_invalidate_styles_on_attribute_changes = b; }
 

Userland/Libraries/LibWeb/DOM/Document.idl

@@ -13,6 +13,8 @@ interface Document : Node {
 
     attribute DOMString cookie;
 
+    readonly attribute USVString referrer;
+
     Element? getElementById(DOMString id);
     HTMLCollection getElementsByName(DOMString name);
     HTMLCollection getElementsByTagName(DOMString tagName);