LibWeb: Start adding support for the <iframe> element! :^)

This patch introduces a bunch of things:

- Subframes (Web::Frame::create_subframe())
- HTMLIFrameElement (loads and owns the hosted Web::Frame)
- LayoutFrame (layout and rendering of the hosted frame)

There's still a huge number of things missing, like scrolling, overflow
handling, event handling, scripting, etc. But we can make a little
iframe in a document and it actually renders another document there.
I think that's pretty cool! :^)

Libraries/LibWeb/DOM/HTMLIFrameElement.cpp

@@ -0,0 +1,96 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <LibCore/ElapsedTimer.h>
+#include <LibGUI/Button.h>
+#include <LibGUI/TextBox.h>
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/Event.h>
+#include <LibWeb/DOM/HTMLFormElement.h>
+#include <LibWeb/DOM/HTMLIFrameElement.h>
+#include <LibWeb/Dump.h>
+#include <LibWeb/Frame.h>
+#include <LibWeb/Layout/LayoutFrame.h>
+#include <LibWeb/Layout/LayoutWidget.h>
+#include <LibWeb/Loader/ResourceLoader.h>
+#include <LibWeb/PageView.h>
+#include <LibWeb/Parser/HTMLDocumentParser.h>
+
+namespace Web {
+
+HTMLIFrameElement::HTMLIFrameElement(Document& document, const FlyString& tag_name)
+    : HTMLElement(document, tag_name)
+{
+    m_hosted_frame = Frame::create_subframe();
+}
+
+HTMLIFrameElement::~HTMLIFrameElement()
+{
+}
+
+RefPtr<LayoutNode> HTMLIFrameElement::create_layout_node(const StyleProperties* parent_style) const
+{
+    auto style = document().style_resolver().resolve_style(*this, parent_style);
+    return adopt(*new LayoutFrame(*this, move(style)));
+}
+
+void HTMLIFrameElement::parse_attribute(const FlyString& name, const String& value)
+{
+    HTMLElement::parse_attribute(name, value);
+
+    if (name == HTML::AttributeNames::src) {
+        load_src(value);
+        return;
+    }
+}
+
+void HTMLIFrameElement::load_src(const String& value)
+{
+    dbg() << "Loading iframe document from " << value;
+    auto url = document().complete_url(value);
+    if (!url.is_valid()) {
+        dbg() << "Actually no I'm not, because the URL is not valid :(";
+        return;
+    }
+
+    ResourceLoader::the().load_sync(
+        url,
+        [&](auto& data, auto& headers) {
+            (void)headers;
+            auto html_source = String::copy(data);
+            HTMLDocumentParser parser(html_source, "utf-8");
+            parser.run(url);
+            m_hosted_frame->set_document(&parser.document());
+
+            dbg() << "The hosted frame now has this DOM:";
+            dump_tree(*m_hosted_frame->document());
+        },
+        [&](auto& error) {
+            dbg() << "<iframe> failed to load: " << error;
+        });
+}
+
+}