Disallow task observers on some BrowserThreads.

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index 8388f59da6d0ab1ea67f99ca4a0ceb8c30e0e269..34aa835d6672bb47cb9228cc1f4b78343fbdc4e5 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -320,6 +320,10 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // 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_;
@@ -458,6 +462,9 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // Whether nesting is allowed.
   bool allow_nesting_ = true;
 
+  // Whether task observers are allowed.
+  bool allow_task_observers_ = true;
+
   DISALLOW_COPY_AND_ASSIGN(MessageLoop);
 };