Break circular dependency between InitializeDeviceDisablingManager and DeviceDisabledScreen

chrome/browser/chromeos/login/screens/device_disabled_screen.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chromeos/login/screens/device_disabled_screen.h"
 
 #include <string>
 
 #include "base/logging.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/browser_process_platform_part.h"
 #include "chrome/browser/chromeos/login/wizard_controller.h"
 
 namespace chromeos {
 
 DeviceDisabledScreen::DeviceDisabledScreen(
     BaseScreenDelegate* base_screen_delegate,
     DeviceDisabledScreenView* view)
     : BaseScreen(base_screen_delegate, OobeScreen::SCREEN_DEVICE_DISABLED),
       view_(view),
       device_disabling_manager_(
           g_browser_process->platform_part()->device_disabling_manager()),
       showing_(false) {
   DCHECK(view_);
   if (view_)
     view_->SetDelegate(this);
+  CHECK(device_disabling_manager_);

[emaxx, 2017/04/12 17:05:49]

I think DCHECK is more suitable there. See the Style Guide:
https://chromium.googlesource.com/chromium/src/+/master/styleguide/c++/c++.md...

[achuithb, 2017/04/12 19:45:14]

Personally I don't see the value of DCHECKing on this line when the next line
will crash anyway.

[Ivan Šandrk, 2017/04/13 13:37:59]

It wasn't really crashing on the next line, it was crashing deeper down the call
stack. I got a bit confused by this at first, and it took me some time to figure
it out (C++ doesn't crash on calling into nullptr object member functions if
they're not virtual, it crashed only when it tried to touch some data).

But if that's what you actually meant by your comment, I will remove it (I still
don't have a gut feeling for (D)CHECKs).

[achuithb, 2017/04/13 18:40:03]

It's fine if you want to leave in the DCHECK. In general, it's not hard to
figure out there's a null ptr dereference when you get a crash stack, and we
don't want to litter our code with DCHECKs before using any pointer.

   device_disabling_manager_->AddObserver(this);
 }
 
 DeviceDisabledScreen::~DeviceDisabledScreen() {
   if (view_)
     view_->SetDelegate(nullptr);
   device_disabling_manager_->RemoveObserver(this);
 }
 
 void DeviceDisabledScreen::Show() {
@@ skipping 26 matching lines @@
   return device_disabling_manager_->disabled_message();
 }
 
 void DeviceDisabledScreen::OnDisabledMessageChanged(
     const std::string& disabled_message) {
   if (view_)
     view_->UpdateMessage(disabled_message);
 }
 
 }  // namespace chromeos