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..8417ce49c0cf25851fbb2fd304b0781bb48ae2ea 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -346,11 +346,13 @@ 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);
+  friend class PendingTaskTest;
 
   // Creates a MessageLoop without binding to a thread.
   // If |type| is TYPE_CUSTOM non-null |pump_factory| must be also given
@@ -450,6 +452,13 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 
   debug::TaskAnnotator task_annotator_;
 
+  // Used to allow creating a breadcrumb of program counters in PostTask.
+  // This variable is only initialized while a task is being executed and is
+  // meant only to store context for creating a backtrace breadcrumb. Do not
+  // attach other semantics to it without thinking through the use caes
+  // thoroughly.
+  const PendingTask* current_pending_task_;
+
   scoped_refptr<internal::IncomingTaskQueue> incoming_task_queue_;
 
   // A task runner which we haven't bound to a thread yet.