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

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 08be568ef565ab67d8cf2db41030872993d65958..544f06fc63871c6b736339687c980367d66e6d90 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
@@ -50,8 +50,8 @@ SigninViewControllerDelegateMac::SigninViewControllerDelegateMac(
 
   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() {}