Extension-created windows should share the creator's BrowingInstance.

chrome/browser/ui/browser_navigator.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 "chrome/browser/ui/browser_navigator.h"
 
 #include <algorithm>
 
 #include "base/command_line.h"
 #include "base/macros.h"
@@ skipping 320 matching lines @@
  private:
   chrome::NavigateParams* params_;
   scoped_ptr<WebContents> target_contents_owner_;
   DISALLOW_COPY_AND_ASSIGN(ScopedTargetContentsOwner);
 };
 
 content::WebContents* CreateTargetContents(const chrome::NavigateParams& params,
                                            const GURL& url) {
   WebContents::CreateParams create_params(
       params.browser->profile(),
-      tab_util::GetSiteInstanceForNewTab(params.browser->profile(), url));
+      params.source_site_instance
+          ? params.source_site_instance
+          : tab_util::GetSiteInstanceForNewTab(params.browser->profile(), url));
   if (params.source_contents) {
     create_params.initial_size =
         params.source_contents->GetContainerBounds().size();
     create_params.created_with_opener = params.created_with_opener;
   }
   if (params.disposition == NEW_BACKGROUND_TAB)
     create_params.initially_hidden = true;
 
 #if defined(USE_AURA)
   if (params.browser->window() &&
@@ skipping 292 matching lines @@
   bool reverse_on_redirect = false;
   content::BrowserURLHandler::GetInstance()->RewriteURLIfNecessary(
       &rewritten_url, browser_context, &reverse_on_redirect);
 
   // Some URLs are mapped to uber subpages. Do not allow them in incognito.
   return !(rewritten_url.scheme() == content::kChromeUIScheme &&
            rewritten_url.host() == chrome::kChromeUIUberHost);
 }
 
 }  // namespace chrome