LibWeb: Implement document.domain getter

The document.domain setter is currently stubbed as that is a doozy to
implement, given how much restrictions there are in place to try and
prevent use of it and potential multi-process implications.

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Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1831,4 +1831,24 @@ JS::NonnullGCPtr<HTML::History> Document::history()
     return *m_history;
 }
 
+// https://html.spec.whatwg.org/multipage/origin.html#dom-document-domain
+String Document::domain() const
+{
+    // 1. Let effectiveDomain be this's origin's effective domain.
+    auto effective_domain = origin().effective_domain();
+
+    // 2. If effectiveDomain is null, then return the empty string.
+    if (!effective_domain.has_value())
+        return String::empty();
+
+    // 3. Return effectiveDomain, serialized.
+    // FIXME: Implement host serialization.
+    return effective_domain.release_value();
+}
+
+void Document::set_domain(String const& domain)
+{
+    dbgln("(STUBBED) Document::set_domain(domain='{}')", domain);
+}
+
 }

Userland/Libraries/LibWeb/DOM/Document.h

@@ -353,6 +353,9 @@ public:
     bool is_initial_about_blank() const { return m_is_initial_about_blank; }
     void set_is_initial_about_blank(bool b) { m_is_initial_about_blank = b; }
 
+    String domain() const;
+    void set_domain(String const& domain);
+
 protected:
     virtual void visit_edges(Cell::Visitor&) override;
 

Userland/Libraries/LibWeb/DOM/Document.idl

@@ -27,6 +27,7 @@ interface Document : Node {
 
     // FIXME: These attributes currently don't do anything.
     [PutForwards=href, LegacyUnforgeable] readonly attribute Location? location;
+    attribute USVString domain;
 
     readonly attribute DOMImplementation implementation;
 

Userland/Libraries/LibWeb/HTML/Origin.h

@@ -92,6 +92,19 @@ public:
         return result.to_string();
     }
 
+    // https://html.spec.whatwg.org/multipage/origin.html#concept-origin-effective-domain
+    Optional<String> effective_domain() const
+    {
+        // 1. If origin is an opaque origin, then return null.
+        if (is_opaque())
+            return Optional<String> {};
+
+        // FIXME: 2. If origin's domain is non-null, then return origin's domain.
+
+        // 3. Return origin's host.
+        return m_host;
+    }
+
     bool operator==(Origin const& other) const { return is_same_origin(other); }
     bool operator!=(Origin const& other) const { return !is_same_origin(other); }