Fix NULL pointer dereferncing in AutoSigninFirstRunDialogView.

This is exactly the same crash and fix as in https://chromium-review.googlesource.com/c/chromium/src/+/951002/

The fix is for 3 different crashes.
- AutoSigninFirstRunDialogView::ControllerGone can trigger some accessibility events. The framework needs the title of the dialog. As a result the controller is dereferenced.
- AutoSigninFirstRunDialogView::Accept and AutoSigninFirstRunDialogView::Cancel. On Mac those events may be triggered when the dialog already disappeared. Therefore, a NULL-check is required before pinging the controller.

Bug: 904154
Change-Id: I58904340599227a5b06eb00a3d46f180bc2864a6
Reviewed-on: https://chromium-review.googlesource.com/c/1333659Reviewed-by: Vaclav Brozek <vabr@chromium.org>
Commit-Queue: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607721}

chrome/browser/ui/views/passwords/auto_signin_first_run_dialog_view.cc

@@ -33,8 +33,10 @@ void AutoSigninFirstRunDialogView::ShowAutoSigninPrompt() {
 }
 
 void AutoSigninFirstRunDialogView::ControllerGone() {
-  controller_ = nullptr;
+  // During Widget::Close() phase some accessibility event may occur. Thus,
+  // |controller_| should be kept around.
   GetWidget()->Close();
+  controller_ = nullptr;
 }
 
 ui::ModalType AutoSigninFirstRunDialogView::GetModalType() const {
@@ -62,12 +64,18 @@ void AutoSigninFirstRunDialogView::WindowClosing() {
 }
 
 bool AutoSigninFirstRunDialogView::Cancel() {
-  controller_->OnAutoSigninTurnOff();
+  // On Mac the button click event may be dispatched after the dialog was
+  // hidden. Thus, the controller can be NULL.
+  if (controller_)
+    controller_->OnAutoSigninTurnOff();
   return true;
 }
 
 bool AutoSigninFirstRunDialogView::Accept() {
-  controller_->OnAutoSigninOK();
+  // On Mac the button click event may be dispatched after the dialog was
+  // hidden. Thus, the controller can be NULL.
+  if (controller_)
+    controller_->OnAutoSigninOK();
   return true;
 }