LibWeb: Add API for setting a document's referrer

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1498,8 +1498,12 @@ bool Document::has_a_style_sheet_that_is_blocking_scripts() const
 
 String Document::referrer() const
 {
-    // FIXME: Return the document's actual referrer.
-    return "";
+    return m_referrer;
+}
+
+void Document::set_referrer(String referrer)
+{
+    m_referrer = referrer;
 }
 
 // https://html.spec.whatwg.org/multipage/browsers.html#fully-active

Userland/Libraries/LibWeb/DOM/Document.h

@@ -64,6 +64,7 @@ public:
     void set_cookie(String const&, Cookie::Source = Cookie::Source::NonHttp);
 
     String referrer() const;
+    void set_referrer(String);
 
     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; }
@@ -483,6 +484,9 @@ private:
     // https://html.spec.whatwg.org/multipage/dom.html#is-initial-about:blank
     bool m_is_initial_about_blank { false };
 
+    // https://html.spec.whatwg.org/multipage/dom.html#the-document's-referrer
+    String m_referrer { "" };
+
     // https://dom.spec.whatwg.org/#concept-document-origin
     HTML::Origin m_origin;
 };