cros: Fix crash when showing user-add screen.

Bug: 855650
Change-Id: Ib9abc4447eb81365439611953e641d30f97cc58d
Reviewed-on: https://chromium-review.googlesource.com/1112344Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569695}

chrome/browser/chromeos/login/ui/login_display.h

@@ -135,7 +135,7 @@ class LoginDisplay {
 
  protected:
   // Login UI delegate (controller).
-  Delegate* delegate_;
+  Delegate* delegate_ = nullptr;
 
   // Parent window, might be used to create dialog windows.
   gfx::NativeWindow parent_window_ = nullptr;

chrome/browser/chromeos/login/ui/login_display_host_webui.cc

@@ -647,8 +647,7 @@ void LoginDisplayHostWebUI::OnStartUserAdding() {
     NOTIMPLEMENTED();
   }
 
-  existing_user_controller_.reset();  // Only one controller in a time.
-  existing_user_controller_.reset(new ExistingUserController(this));
+  CreateExistingUserController();
 
   if (!signin_screen_controller_.get()) {
     signin_screen_controller_.reset(new SignInScreenController(GetOobeUI()));
@@ -698,9 +697,7 @@ void LoginDisplayHostWebUI::OnStartSignInScreen(
   }
 
   DVLOG(1) << "Starting sign in screen";
-  existing_user_controller_.reset();  // Only one controller in a time.
-  existing_user_controller_.reset(new ExistingUserController(this));
-  login_display_->set_delegate(existing_user_controller_.get());
+  CreateExistingUserController();
 
   if (!signin_screen_controller_.get()) {
     signin_screen_controller_.reset(new SignInScreenController(GetOobeUI()));
@@ -1138,6 +1135,13 @@ void LoginDisplayHostWebUI::OnLoginPromptVisible() {
   TryToPlayOobeStartupSound();
 }
 
+void LoginDisplayHostWebUI::CreateExistingUserController() {
+  // There can only be one |ExistingUserController| instance at a time.
+  existing_user_controller_.reset();
+  existing_user_controller_.reset(new ExistingUserController(this));
+  login_display_->set_delegate(existing_user_controller_.get());
+}
+
 // static
 void LoginDisplayHostWebUI::DisableRestrictiveProxyCheckForTest() {
   default_host()

chrome/browser/chromeos/login/ui/login_display_host_webui.h

@@ -177,6 +177,9 @@ class LoginDisplayHostWebUI : public LoginDisplayHostCommon,
   // Called when login-prompt-visible signal is caught.
   void OnLoginPromptVisible();
 
+  // Creates or recreates |existing_user_controller_|.
+  void CreateExistingUserController();
+
   // Sign in screen controller.
   std::unique_ptr<ExistingUserController> existing_user_controller_;