Use the same browser instance in the sync confirmation dialog.

chrome/browser/ui/cocoa/profiles/signin_view_controller_delegate_mac.mm

 // Copyright 2016 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/profiles/signin_view_controller_delegate_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "chrome/app/chrome_command_ids.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_avatar_icon_util.h"
 #include "chrome/browser/signin/signin_promo.h"
 #include "chrome/browser/ui/browser.h"
 #import "chrome/browser/ui/cocoa/browser_window_utils.h"
 #include "chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_sheet.h"
 #include "chrome/browser/ui/cocoa/constrained_window/constrained_window_custom_window.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
+#include "chrome/browser/ui/webui/signin/sync_confirmation_ui.h"
 #include "chrome/common/url_constants.h"
 #include "components/signin/core/common/profile_management_switches.h"
 #include "content/public/browser/native_web_keyboard_event.h"
 #include "content/public/browser/render_widget_host_view.h"
 #include "content/public/browser/web_contents.h"
 
 namespace {
 
 // Dimensions of the web contents containing the old-style signin flow with the
 // username and password challenge on the same form.
@@ skipping 86 matching lines @@
 // static
 std::unique_ptr<content::WebContents>
 SigninViewControllerDelegateMac::CreateSyncConfirmationWebContents(
     Profile* profile) {
   std::unique_ptr<content::WebContents> web_contents(
       content::WebContents::Create(
           content::WebContents::CreateParams(profile)));
   web_contents->GetController().LoadURL(
       GURL(chrome::kChromeUISyncConfirmationURL), content::Referrer(),
       ui::PAGE_TRANSITION_AUTO_TOPLEVEL, std::string());
+  SyncConfirmationUI* sync_confirmation_ui = static_cast<SyncConfirmationUI*>(
+      web_contents->GetWebUI()->GetController());
+  sync_confirmation_ui->InitializeMessageHandlerWithBrowser(browser);
 
   NSView* webview = web_contents->GetNativeView();
   [webview setFrameSize:NSMakeSize(
                             kModalDialogWidth,
                             GetSyncConfirmationDialogPreferredHeight(profile))];
 
   return web_contents;
 }
 
 // static
@@ skipping 124 matching lines @@
 SigninViewControllerDelegate::CreateSigninErrorDelegate(
     SigninViewController* signin_view_controller,
     Browser* browser) {
   return new SigninViewControllerDelegateMac(
       signin_view_controller,
       SigninViewControllerDelegateMac::CreateSigninErrorWebContents(
           browser->profile()),
       browser, NSMakeRect(0, 0, kModalDialogWidth, kSigninErrorDialogHeight),
       ui::MODAL_TYPE_WINDOW, true /* wait_for_size */);
 }