Implement a new flow event API that allows binding flow events and regular events.

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 19df8cb39e5129b9c02987f919b8bdb7cc1d7760..e456089d95dce17916c1ffe8a015e08f9d9fcd95 100644
--- a/base/debug/task_annotator.cc
+++ b/base/debug/task_annotator.cc
@@ -20,6 +20,9 @@ TaskAnnotator::~TaskAnnotator() {
 
 void TaskAnnotator::DidQueueTask(const char* queue_function,
                                  const PendingTask& pending_task) {
+  TRACE_EVENT_WITH_FLOW0("toplevel", "TaskAnnotator::DidQueueTask",
+                         TRACE_ID_MANGLE(GetTaskTraceID(pending_task)),
+                         TRACE_EVENT_FLAG_FLOW_OUT);
   TRACE_EVENT_FLOW_BEGIN0(TRACE_DISABLED_BY_DEFAULT("toplevel.flow"),
                           queue_function,
                           TRACE_ID_MANGLE(GetTaskTraceID(pending_task)));
@@ -33,24 +36,21 @@ void TaskAnnotator::RunTask(const char* queue_function,
   tracked_objects::Duration queue_duration =
       stopwatch.StartTime() - pending_task.EffectiveTimePosted();
 
-  TRACE_EVENT_FLOW_END1(TRACE_DISABLED_BY_DEFAULT("toplevel.flow"),
-                        queue_function,
-                        TRACE_ID_MANGLE(GetTaskTraceID(pending_task)),
-                        "queue_duration",
-                        queue_duration.InMilliseconds());
-
   // When tracing memory for posted tasks it's more valuable to attribute the
   // memory allocations to the source function than generically to the task
   // runner.
-  TRACE_EVENT_WITH_MEMORY_TAG2(
-      "toplevel",
-      run_function,
+  TRACE_EVENT_WITH_MEMORY_TAG_WITH_FLOW2(
+      "toplevel", run_function,
       pending_task.posted_from.function_name(),  // Name for memory tracking.
-      "src_file",
-      pending_task.posted_from.file_name(),
-      "src_func",
+      TRACE_ID_MANGLE(GetTaskTraceID(pending_task)), TRACE_EVENT_FLAG_FLOW_IN,
+      "src_file", pending_task.posted_from.file_name(), "src_func",
       pending_task.posted_from.function_name());
 
+  TRACE_EVENT_FLOW_END1(TRACE_DISABLED_BY_DEFAULT("toplevel.flow"),
+                        queue_function,
+                        TRACE_ID_MANGLE(GetTaskTraceID(pending_task)),
+                        "queue_duration", queue_duration.InMilliseconds());
+
   // 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