Add logging to CHECK failure during curtain-mode on Mac.

remoting/host/curtain_mode_mac.cc

 // 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.
 
 #include "remoting/host/curtain_mode.h"
 
 #include <ApplicationServices/ApplicationServices.h>
 #include <Carbon/Carbon.h>
 #include <Security/Security.h>
 #include <unistd.h>
@@ skipping 126 matching lines @@
   base::ScopedCFTypeRef<CFDictionaryRef> session(
       CGSessionCopyCurrentDictionary());
 
   // CGSessionCopyCurrentDictionary has been observed to return nullptr in some
   // cases. Once the system is in this state, curtain mode will fail as the
   // CGSession command thinks the session is not attached to the console. The
   // only known remedy is logout or reboot. Since we're not sure what causes
   // this, or how common it is, a crash report is useful in this case (note
   // that the connection would have to be refused in any case, so this is no
   // loss of functionality).
-  CHECK(session != nullptr);
+  CHECK(session != nullptr)
+      << "Error activating curtain-mode: "
+      << "CGSessionCopyCurrentDictionary() returned NULL. "
+      << "Logging out and back in should resolve this error.";
 
   const void* on_console = CFDictionaryGetValue(session,
                                                 kCGSessionOnConsoleKey);
   const void* logged_in = CFDictionaryGetValue(session, kCGSessionLoginDoneKey);
   if (logged_in == kCFBooleanTrue && on_console == kCFBooleanTrue) {
     pid_t child = fork();
     if (child == 0) {
       execl(kCGSessionPath, kCGSessionPath, "-suspend", nullptr);
       _exit(1);
     } else if (child > 0) {
@@ skipping 100 matching lines @@
 // static
 scoped_ptr<CurtainMode> CurtainMode::Create(
     scoped_refptr<base::SingleThreadTaskRunner> caller_task_runner,
     scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner,
     base::WeakPtr<ClientSessionControl> client_session_control) {
   return make_scoped_ptr(new CurtainModeMac(
       caller_task_runner, ui_task_runner, client_session_control));
 }
 
 }  // namespace remoting