LibWeb: Defer creation of subframes until host element is connected

This allows parsing of document fragments with "<iframe>" to construct
the iframe element without requiring that the fragment have a frame.

Userland/Libraries/LibWeb/HTML/FrameHostElement.cpp

@@ -41,6 +41,15 @@ FrameHostElement::~FrameHostElement()
 {
 }
 
+void FrameHostElement::inserted_into(Node& parent)
+{
+    HTMLElement::inserted_into(parent);
+    if (!is_connected())
+        return;
+    if (auto* frame = document().frame())
+        m_content_frame = Frame::create_subframe(*this, frame->main_frame());
+}
+
 Origin FrameHostElement::content_origin() const
 {
     if (!m_content_frame || !m_content_frame->document())

Userland/Libraries/LibWeb/HTML/FrameHostElement.h

@@ -45,6 +45,8 @@ public:
 
     void content_frame_did_load(Badge<FrameLoader>);
 
+    virtual void inserted_into(Node&) override;
+
 protected:
     RefPtr<Frame> m_content_frame;
 };

Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.cpp

@@ -35,8 +35,6 @@ namespace Web::HTML {
 HTMLIFrameElement::HTMLIFrameElement(DOM::Document& document, QualifiedName qualified_name)
     : FrameHostElement(document, move(qualified_name))
 {
-    VERIFY(document.frame());
-    m_content_frame = Frame::create_subframe(*this, document.frame()->main_frame());
 }
 
 HTMLIFrameElement::~HTMLIFrameElement()
@@ -56,8 +54,18 @@ void HTMLIFrameElement::parse_attribute(const FlyString& name, const String& val
         load_src(value);
 }
 
+void HTMLIFrameElement::inserted_into(Node& parent)
+{
+    FrameHostElement::inserted_into(parent);
+    if (is_connected())
+        load_src(attribute(HTML::AttributeNames::src));
+}
+
 void HTMLIFrameElement::load_src(const String& value)
 {
+    if (!m_content_frame)
+        return;
+
     auto url = document().complete_url(value);
     if (!url.is_valid()) {
         dbgln("iframe failed to load URL: Invalid URL: {}", value);

Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.h

@@ -40,6 +40,7 @@ public:
     virtual RefPtr<Layout::Node> create_layout_node() override;
 
 private:
+    virtual void inserted_into(Node&) override;
     virtual void parse_attribute(const FlyString& name, const String& value) override;
 
     void load_src(const String&);