[Signin] Remove the close button from ProfileSigninConfirmationDialog

Modal dialogs shouldn't have the X close button. This button is
redundant as the dialog already has the Cancel button.

Screenshot: https://drive.google.com/file/d/1eYpxgg4HuPjdpXs2Ea6FeRp3KPGFRWnF/view?usp=sharing

Fixed: 1122471
Change-Id: I179e34e8bceac980d9835332e2d7dc521a67f4de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2387015
Auto-Submit: Alex Ilin <alexilin@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803418}

chrome/browser/ui/views/sync/profile_signin_confirmation_dialog_views.cc

@@ -52,6 +52,7 @@ ProfileSigninConfirmationDialogViews::ProfileSigninConfirmationDialogViews(
       username_(username),
       delegate_(std::move(delegate)),
       prompt_for_new_profile_(prompt_for_new_profile) {
+  SetShowCloseButton(false);
   SetTitle(IDS_ENTERPRISE_SIGNIN_TITLE);
   SetDefaultButton(ui::DIALOG_BUTTON_NONE);
   SetButtonLabel(