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 e968d8a88ee1e32e88608b526ba940cafd5e6df0..dfdb070f68f93161b6cf76b6884028946401be80 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
@@ -81,8 +81,8 @@ 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() {}