Record async "task backtraces"

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 91a7b1d33268d470ee4220202dd45734bc3d01ec..1b6afcd590ac5d91ce7b56e9fe026863792c0b5e 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -346,10 +346,11 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   void BindToCurrentThread();
 
  private:
-  friend class RunLoop;
   friend class internal::IncomingTaskQueue;
+  friend class RunLoop;
   friend class ScheduleWorkTest;
   friend class Thread;
+  friend struct PendingTask;
   FRIEND_TEST_ALL_PREFIXES(MessageLoopTest, DeleteUnboundLoop);
 
   // Creates a MessageLoop without binding to a thread.
@@ -450,6 +451,10 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 
   debug::TaskAnnotator task_annotator_;
 
+  // Used to allow creating a breadcrumb of program counters in PostTask.
+  // Be very careful when using this variable.

[danakj, 2017/02/07 23:16:46]

careful in what way, what are the risks?

[awong, 2017/02/08 01:55:50]

rewrote comment.

+  const PendingTask* current_pending_task_;
+
   scoped_refptr<internal::IncomingTaskQueue> incoming_task_queue_;
 
   // A task runner which we haven't bound to a thread yet.