Add debug code to help understand SEGV in ~ErrorScreen.

chrome/browser/chromeos/login/screens/error_screen.h

 // Copyright (c) 2012 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_ERROR_SCREEN_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_ERROR_SCREEN_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 85 matching lines @@
     parent_screen_ = parent_screen;
   }
   OobeDisplay::Screen parent_screen() const { return parent_screen_; }
 
  private:
   // Handles the response of an ownership check and starts the guest session if
   // applicable.
   void StartGuestSessionAfterOwnershipCheck(
       DeviceSettingsService::OwnershipStatus ownership_status);
 
+  volatile void* canary_1_;  // For debugging of https://crbug.com/396557.

[ygorshenin1, 2014/08/13 08:36:51]

As you're just reading pointer's value, not a value pointers point to, you can
simply replace "void*" by "long" - it has the same size as a pointer but less
obfuscating.

[Thiemo Nagel, 2014/08/13 09:38:48]

The sizes of pointer and long depend on the data model.  Although I'm inclined
to believe that sizeof(void*)==sizeof(long) for all architectures supported by
Chromium OS, technically there is no guarantee for that and thus I think that
it's actually less obfuscating to use void* here.  (For example Windows 64bit
uses the LLP64 data model where sizeof(void*)==8 and sizeof(long)==4.)

What do you think?

   ErrorScreenActor* actor_;
+  volatile void* canary_2_;
 
   OobeDisplay::Screen parent_screen_;
 
   scoped_ptr<LoginPerformer> guest_login_performer_;
 
   base::WeakPtrFactory<ErrorScreen> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ErrorScreen);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_ERROR_SCREEN_H_