Disallow task observers on some BrowserThreads.

base/message_loop/message_loop.h

 // 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.
 
 #ifndef BASE_MESSAGE_LOOP_MESSAGE_LOOP_H_
 #define BASE_MESSAGE_LOOP_MESSAGE_LOOP_H_
 
 #include <memory>
 #include <queue>
 #include <string>
@@ skipping 302 matching lines @@
   // tasks.
   debug::TaskAnnotator* task_annotator() { return &task_annotator_; }
 
   // Runs the specified PendingTask.
   void RunTask(const PendingTask& pending_task);
 
   // Disallow nesting. After this is called, running a nested RunLoop or calling
   // Add/RemoveNestingObserver() on this MessageLoop will crash.
   void DisallowNesting() { allow_nesting_ = false; }
 
+  // Disallow task observers. After this is called, calling
+  // Add/RemoveTaskObserver() on this MessageLoop will crash.
+  void DisallowTaskObservers() { allow_task_observers_ = false; }
+
   //----------------------------------------------------------------------------
  protected:
   std::unique_ptr<MessagePump> pump_;
 
   using MessagePumpFactoryCallback = Callback<std::unique_ptr<MessagePump>()>;
 
   // Common protected constructor. Other constructors delegate the
   // initialization to this constructor.
   // A subclass can invoke this constructor to create a message_loop of a
   // specific type with a custom loop. The implementation does not call
@@ skipping 118 matching lines @@
   scoped_refptr<SingleThreadTaskRunner> task_runner_;
   std::unique_ptr<ThreadTaskRunnerHandle> thread_task_runner_handle_;
 
   // Id of the thread this message loop is bound to. Initialized once when the
   // MessageLoop is bound to its thread and constant forever after.
   PlatformThreadId thread_id_;
 
   // Whether nesting is allowed.
   bool allow_nesting_ = true;
 
+  // Whether task observers are allowed.
+  bool allow_task_observers_ = true;
+
   DISALLOW_COPY_AND_ASSIGN(MessageLoop);
 };
 
 #if !defined(OS_NACL)
 
 //-----------------------------------------------------------------------------
 // MessageLoopForUI extends MessageLoop with methods that are particular to a
 // MessageLoop instantiated with TYPE_UI.
 //
 // This class is typically used like so:
@@ skipping 127 matching lines @@
 
 // Do not add any member variables to MessageLoopForIO!  This is important b/c
 // MessageLoopForIO is often allocated via MessageLoop(TYPE_IO).  Any extra
 // data that you need should be stored on the MessageLoop's pump_ instance.
 static_assert(sizeof(MessageLoop) == sizeof(MessageLoopForIO),
               "MessageLoopForIO should not have extra member variables");
 
 }  // namespace base
 
 #endif  // BASE_MESSAGE_LOOP_MESSAGE_LOOP_H_