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/Layout/LayoutFrame.cpp

@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2018-2020, 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 <LibGUI/Painter.h>
+#include <LibGUI/ScrollBar.h>
+#include <LibGUI/Widget.h>
+#include <LibGfx/Font.h>
+#include <LibGfx/StylePainter.h>
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/Frame.h>
+#include <LibWeb/Layout/LayoutDocument.h>
+#include <LibWeb/Layout/LayoutFrame.h>
+#include <LibWeb/PageView.h>
+
+namespace Web {
+
+LayoutFrame::LayoutFrame(const Element& element, NonnullRefPtr<StyleProperties> style)
+    : LayoutReplaced(element, move(style))
+{
+}
+
+LayoutFrame::~LayoutFrame()
+{
+}
+
+void LayoutFrame::layout(LayoutMode layout_mode)
+{
+    set_has_intrinsic_width(true);
+    set_has_intrinsic_height(true);
+    // FIXME: Do proper error checking, etc.
+    bool ok;
+    set_intrinsic_width(node().attribute(HTML::AttributeNames::width).to_int(ok));
+    set_intrinsic_height(node().attribute(HTML::AttributeNames::height).to_int(ok));
+
+    LayoutReplaced::layout(layout_mode);
+}
+
+void LayoutFrame::render(RenderingContext& context)
+{
+    LayoutReplaced::render(context);
+
+    context.painter().save();
+    auto old_viewport_rect = context.viewport_rect();
+
+    context.painter().add_clip_rect(enclosing_int_rect(rect()));
+    context.painter().translate(x(), y());
+
+    context.set_viewport_rect({ {}, node().hosted_frame()->size() });
+    node().hosted_frame()->document()->layout_node()->render(context);
+
+    context.set_viewport_rect(old_viewport_rect);
+    context.painter().restore();
+}
+
+void LayoutFrame::did_set_rect()
+{
+    LayoutReplaced::did_set_rect();
+
+    ASSERT(node().hosted_frame());
+    node().hosted_frame()->set_size(Gfx::Size(rect().width(), rect().height()));
+
+    node().hosted_frame()->document()->layout();
+}
+
+}