LibWeb: Attach DOM::Document to its frame before parsing

FrameLoader now begins by constructing a DOM::Document, and then builds
a document tree inside it based on the MIME type. For text/html we pass
control to the HTMLDocumentParser as before.

This gives us access to things like window.alert() during parsing.

Fixes #3973.

Libraries/LibWeb/Page/Frame.cpp

@@ -159,6 +159,9 @@ void Frame::scroll_to_anchor(const String& fragment)
         }
     }
 
+    // FIXME: This is overly aggressive and should be something more like a "update_layout_if_needed()"
+    document()->force_layout();
+
     if (!element || !element->layout_node())
         return;