Fix a regression where the tab-modal signin flow wasn't shown.

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/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_avatar_icon_util.h"
@@ skipping 32 matching lines @@
     content::WebContents* host_web_contents,
     NSRect frame)
     : SigninViewControllerDelegate(signin_view_controller, web_contents.get()),
       web_contents_(std::move(web_contents)),
       window_(
           [[ConstrainedWindowCustomWindow alloc] initWithContentRect:frame]) {
   window_.get().contentView = web_contents_->GetNativeView();
 
   base::scoped_nsobject<CustomConstrainedWindowSheet> sheet(
       [[CustomConstrainedWindowSheet alloc] initWithCustomWindow:window_]);
-  constrained_window_.reset(
-      new ConstrainedWindowMac(this, host_web_contents, sheet));
+  constrained_window_ =
+      CreateAndShowWebModalDialogMac(this, host_web_contents, sheet);
 }
 
 SigninViewControllerDelegateMac::~SigninViewControllerDelegateMac() {}
 
 void SigninViewControllerDelegateMac::OnConstrainedWindowClosed(
     ConstrainedWindowMac* window) {
   ResetSigninViewControllerDelegate();
   delete this;
 }
 
@@ skipping 72 matching lines @@
     SigninViewController* signin_view_controller,
     Browser* browser) {
   return new SigninViewControllerDelegateMac(
       signin_view_controller,
       SigninViewControllerDelegateMac::CreateSyncConfirmationWebContents(
           browser->profile()),
       browser->tab_strip_model()->GetActiveWebContents(),
       NSMakeRect(0, 0, kSyncConfirmationDialogWidth,
                  kSyncConfirmationDialogHeight));
 }