[Extensions UI] Don't show the (x) button in settings overridden dialogs

The latest UX style indicates we shouldn't show a (x) button in modal
dialogs. Update the extension settings overridden dialogs to match.

Note that the dialog can still be dismissed without a decision by
- hitting the escape key (which triggers the "closed" callback)
- closing the parent widget

Bug: 1079364
Change-Id: I4dea1c41656f76e5951267cfd3c2459ad9e148e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2247352Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779339}

chrome/browser/ui/views/extensions/settings_overridden_dialog_view.cc

@@ -43,6 +43,12 @@ SettingsOverriddenDialogView::SettingsOverriddenDialogView(
   SetCancelCallback(make_result_callback(DialogResult::kKeepNewSettings));
   SetCloseCallback(make_result_callback(DialogResult::kDialogDismissed));
 
+  // Modals shouldn't show a close button according to the latest style
+  // guidelines. Note the dialog can still be dismissed by user action via the
+  // escape key (in addition to closing automatically if the parent widget
+  // is destroyed).
+  SetShowCloseButton(false);
+
   SettingsOverriddenDialogController::ShowParams show_params =
       controller_->GetShowParams();