Use the outermost embedder WebContents in ConstrainedWindowMac.

chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.mm

 // 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 "chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.h"
 
 #include "base/memory/scoped_ptr.h"
 #include "base/logging.h"
 #import "chrome/browser/ui/cocoa/constrained_window/constrained_window_sheet.h"
 #import "chrome/browser/ui/cocoa/single_web_contents_dialog_manager_cocoa.h"
 #include "components/web_modal/web_contents_modal_dialog_manager.h"
 #include "content/public/browser/browser_thread.h"
 #include "extensions/browser/guest_view/guest_view_base.h"
 
 using web_modal::WebContentsModalDialogManager;
 
 ConstrainedWindowMac::ConstrainedWindowMac(
     ConstrainedWindowMacDelegate* delegate,
     content::WebContents* web_contents,
     id<ConstrainedWindowSheet> sheet)
     : delegate_(delegate) {
   DCHECK(sheet);
-  extensions::GuestViewBase* guest_view =
-      extensions::GuestViewBase::FromWebContents(web_contents);
-  // For embedded WebContents, use the embedder's WebContents for constrained
-  // window.
-  web_contents = guest_view && guest_view->embedder_web_contents() ?
-                    guest_view->embedder_web_contents() : web_contents;
+
+  // |web_contents| may be embedded within a chain of nested GuestViews. If it
+  // is, follow the chain of embedders to the outermost WebContents and use it.
+  while (extensions::GuestViewBase* guest_view =

[Fady Samuel, 2015/03/26 01:43:02]

Drive by comment: This pattern has started emerging in a couple of places. I
wonder if it makes sense to put it in GuestViewBase?

It could either be a member method or a static method:

guest->GetTopMostWebContents()

or 

GuestViewBase::GetTopMostWebContents(web_contents)

+             extensions::GuestViewBase::FromWebContents(web_contents)) {
+    if (!guest_view->embedder_web_contents())
+      break;
+    web_contents = guest_view->embedder_web_contents();
+  }
 
   auto manager = WebContentsModalDialogManager::FromWebContents(web_contents);
   scoped_ptr<SingleWebContentsDialogManagerCocoa> native_manager(
       new SingleWebContentsDialogManagerCocoa(this, sheet, manager));
   manager->ShowDialogWithManager([sheet sheetWindow], native_manager.Pass());
 }
 
 ConstrainedWindowMac::~ConstrainedWindowMac() {
   CHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   DCHECK(!manager_);
 }
 
 void ConstrainedWindowMac::CloseWebContentsModalDialog() {
   if (manager_)
     manager_->Close();
 }
 
 void ConstrainedWindowMac::OnDialogClosing() {
   if (delegate_)
     delegate_->OnConstrainedWindowClosed(this);
 }