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 a55fa518ea070e61ae4d4d23139731e86db5ac15..7f3fccd8827c934f04b42debf5017d407b3bf3fa 100644
--- a/base/pending_task.h
+++ b/base/pending_task.h
@@ -5,6 +5,7 @@
 #ifndef BASE_PENDING_TASK_H_
 #define BASE_PENDING_TASK_H_
 
+#include <array>
 #include <queue>
 
 #include "base/base_export.h"
@@ -37,6 +38,9 @@ struct BASE_EXPORT PendingTask : public TrackingInfo {
   // The site this PendingTask was posted from.
   tracked_objects::Location posted_from;
 
+  // Task backtrace.
+  std::array<const void*, 4> task_backtrace;
+
   // Secondary sort key for run time.
   int sequence_num;