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

https://codereview.chromium.org/1781433002 changed some of the
constrained_window code and updated some callers, but not the tab-modal
signin flow. The modal was being created but not shown.

BUG=593091,217034

Review URL: https://codereview.chromium.org/1815433002

Cr-Commit-Position: refs/heads/master@{#381778}

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() {}