Use the same browser instance in the sync confirmation dialog.

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

Index: chrome/browser/ui/cocoa/profiles/signin_view_controller_delegate_mac.mm
diff --git a/chrome/browser/ui/cocoa/profiles/signin_view_controller_delegate_mac.mm b/chrome/browser/ui/cocoa/profiles/signin_view_controller_delegate_mac.mm
index 9741d0f197f245a1351ec5fa18b332f95cadae76..69d908fc77b61dfefa88b1a0a616d17e847cd4fb 100644
--- a/chrome/browser/ui/cocoa/profiles/signin_view_controller_delegate_mac.mm
+++ b/chrome/browser/ui/cocoa/profiles/signin_view_controller_delegate_mac.mm
@@ -15,6 +15,7 @@
 #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"
@@ -121,6 +122,9 @@ SigninViewControllerDelegateMac::CreateSyncConfirmationWebContents(
   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(