Record async "task backtraces"

base/message_loop/message_loop.cc

Index: base/message_loop/message_loop.cc
diff --git a/base/message_loop/message_loop.cc b/base/message_loop/message_loop.cc
index eb0f968849485ea73ef8a1aab9094bb9527af403..cacc26f0884ac3154aebc116795d5c715f1947e1 100644
--- a/base/message_loop/message_loop.cc
+++ b/base/message_loop/message_loop.cc
@@ -126,7 +126,8 @@ MessageLoop::MessageLoop(Type type)
       os_modal_loop_(false),
 #endif  // OS_WIN
       message_histogram_(NULL),
-      run_loop_(NULL) {
+      run_loop_(NULL),
+      current_pending_task_(NULL) {
   Init();
 
   pump_ = CreateMessagePumpForType(type).Pass();
@@ -426,6 +427,7 @@ bool MessageLoop::ProcessNextDelayedNonNestableTask() {
 
 void MessageLoop::RunTask(const PendingTask& pending_task) {
   DCHECK(nestable_tasks_allowed_);
+  current_pending_task_ = &pending_task;
 
 #if defined(OS_WIN)
   if (pending_task.is_high_res) {
@@ -447,6 +449,8 @@ void MessageLoop::RunTask(const PendingTask& pending_task) {
                     DidProcessTask(pending_task));
 
   nestable_tasks_allowed_ = true;
+
+  current_pending_task_ = NULL;
 }
 
 bool MessageLoop::DeferOrRunPendingTask(const PendingTask& pending_task) {