Disallow nesting 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 4f3250c3b7f1b164d984dc0e4dcee28866092015..8388f59da6d0ab1ea67f99ca4a0ceb8c30e0e269 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -316,6 +316,10 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // 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; }
+
   //----------------------------------------------------------------------------
  protected:
   std::unique_ptr<MessagePump> pump_;
@@ -451,6 +455,9 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // MessageLoop is bound to its thread and constant forever after.
   PlatformThreadId thread_id_;
 
+  // Whether nesting is allowed.
+  bool allow_nesting_ = true;
+
   DISALLOW_COPY_AND_ASSIGN(MessageLoop);
 };