Record async "task backtraces"

base/debug/task_annotator.cc

Index: base/debug/task_annotator.cc
diff --git a/base/debug/task_annotator.cc b/base/debug/task_annotator.cc
index 3591fd6c80fc19016b4ed195233222fbc9c70808..d702599e593602b6dda2f472b7975cc5e7d30f9a 100644
--- a/base/debug/task_annotator.cc
+++ b/base/debug/task_annotator.cc
@@ -52,13 +52,15 @@ void TaskAnnotator::RunTask(const char* queue_function,
       "src_func",
       pending_task.posted_from.function_name());
 
-  // Before running the task, store the program counter where it was posted
-  // and deliberately alias it to ensure it is on the stack if the task
-  // crashes. Be careful not to assume that the variable itself will have the
-  // expected value when displayed by the optimizer in an optimized build.
-  // Look at a memory dump of the stack.
-  const void* program_counter = pending_task.posted_from.program_counter();
-  debug::Alias(&program_counter);
+  // Before running the task, store the task backtrace with the chain of
+  // PostTasks that resulted in this call and deliberately alias it to ensure
+  // it is on the stack if the task crashes. Be careful not to assume that the
+  // variable itself will have the expected value when displayed by the
+  // optimizer in an optimized build. Look at a memory dump of the stack.
+  const void* task_backtrace[4];
+  memcpy(&task_backtrace[0], &pending_task.task_backtrace[0],
+         sizeof(task_backtrace));
+  debug::Alias(&task_backtrace);
 
   pending_task.task.Run();