Track where WebContents are created in order to better understand issue.

components/constrained_window/constrained_window_views.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/constrained_window/constrained_window_views.h"
 
 #include <algorithm>
 
+#include "base/debug/alias.h"
+#include "base/debug/stack_trace.h"
 #include "components/constrained_window/constrained_window_views_client.h"
 #include "components/guest_view/browser/guest_view_base.h"
 #include "components/web_modal/web_contents_modal_dialog_host.h"
 #include "components/web_modal/web_contents_modal_dialog_manager.h"
 #include "components/web_modal/web_contents_modal_dialog_manager_delegate.h"
 #include "ui/views/border.h"
 #include "ui/views/widget/widget.h"
 #include "ui/views/widget/widget_observer.h"
 #include "ui/views/window/dialog_delegate.h"
 
@@ skipping 120 matching lines @@
   content::WebContents* web_contents =
       guest_view::GuestViewBase::GetTopLevelWebContents(initiator_web_contents);
   views::Widget* widget = CreateWebModalDialogViews(dialog, web_contents);
   web_modal::WebContentsModalDialogManager::FromWebContents(web_contents)
       ->ShowModalDialog(widget->GetNativeWindow());
   return widget;
 }
 
 views::Widget* CreateWebModalDialogViews(views::WidgetDelegate* dialog,
                                          content::WebContents* web_contents) {
+  // Temporary to track down http://crbug.com/538612
+  base::debug::StackTrace contents_stack_trace = web_contents->stack_trace();

[ncarter (slow), 2015/11/16 23:54:45]

Have you tried just aliasing the value of web_contents->GetDelegate()?

That should tell you (say, by looking at the vtable __vfptr under VisualStudio)
which subclass of content::WebContentsDelegate this WebContents was created for.

These are the subclasses:
https://code.google.com/p/chromium/codesearch#search/&q=%22public%20content::...

[hcarmona, 2015/11/17 03:12:51]

This should work if the delegates always have their web contents created
in the same way. I got Browser as the delegate when I tried this out
with the print preview dialog on a regular window. Does that seem right?

[Mike Wittman, 2015/11/17 16:24:06]

I don't know much about WebContentsDelegate, but sounds plausible to me.

+  base::debug::Alias(&contents_stack_trace);
+  CHECK(
+      web_modal::WebContentsModalDialogManager::FromWebContents(web_contents));
+
   DCHECK_EQ(ui::MODAL_TYPE_CHILD, dialog->GetModalType());
   return views::DialogDelegate::CreateDialogWidget(
       dialog, nullptr,
       web_modal::WebContentsModalDialogManager::FromWebContents(web_contents)
           ->delegate()
           ->GetWebContentsModalDialogHost()
           ->GetHostView());
 }
 
 views::Widget* CreateBrowserModalDialogViews(views::DialogDelegate* dialog,
@@ skipping 25 matching lines @@
     DCHECK_EQ(parent_view, host->GetHostView());
     ModalDialogHostObserver* dialog_host_observer =
         new WidgetModalDialogHostObserverViews(
             host, widget, kWidgetModalDialogHostObserverViewsKey);
     dialog_host_observer->OnPositionRequiresUpdate();
   }
   return widget;
 }
 
 }  // namespace constrained window