Record async "task backtraces"

base/pending_task.h

Index: base/pending_task.h
diff --git a/base/pending_task.h b/base/pending_task.h
index fddfc868fe7dc3227df4287f9fb33752cb90f419..7571f57eb27d91b8c64e518c2ae5d4ecbc739de9 100644
--- a/base/pending_task.h
+++ b/base/pending_task.h
@@ -19,8 +19,10 @@ namespace base {
 // for use by classes that queue and execute tasks.
 struct BASE_EXPORT PendingTask : public TrackingInfo {
   PendingTask(const tracked_objects::Location& posted_from,
+              const PendingTask* parent_task,
               const Closure& task);
   PendingTask(const tracked_objects::Location& posted_from,
+              const PendingTask* parent_task,
               const Closure& task,
               TimeTicks delayed_run_time,
               bool nestable);
@@ -35,6 +37,9 @@ struct BASE_EXPORT PendingTask : public TrackingInfo {
   // The site this PendingTask was posted from.
   tracked_objects::Location posted_from;
 
+  // Task backtrace.
+  const void* task_backtrace[4];
+
   // Secondary sort key for run time.
   int sequence_num;