Disallow task observers on some BrowserThreads.

base/message_loop/message_loop.cc

 // Copyright 2013 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 "base/message_loop/message_loop.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 260 matching lines @@
 bool MessageLoop::NestableTasksAllowed() const {
   return nestable_tasks_allowed_;
 }
 
 bool MessageLoop::IsNested() {
   return run_loop_->run_depth_ > 1;
 }
 
 void MessageLoop::AddTaskObserver(TaskObserver* task_observer) {
   DCHECK_EQ(this, current());
+  CHECK(allow_task_observers_);
   task_observers_.AddObserver(task_observer);
 }
 
 void MessageLoop::RemoveTaskObserver(TaskObserver* task_observer) {
   DCHECK_EQ(this, current());
+  CHECK(allow_task_observers_);
   task_observers_.RemoveObserver(task_observer);
 }
 
 bool MessageLoop::is_running() const {
   DCHECK_EQ(this, current());
   return run_loop_ != NULL;
 }
 
 bool MessageLoop::HasHighResolutionTasks() {
   return incoming_task_queue_->HasHighResolutionTasks();
@@ skipping 356 matching lines @@
       persistent,
       mode,
       controller,
       delegate);
 }
 #endif
 
 #endif  // !defined(OS_NACL_SFI)
 
 }  // namespace base