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

base/threading/sequenced_worker_pool.cc

Index: base/threading/sequenced_worker_pool.cc
diff --git a/base/threading/sequenced_worker_pool.cc b/base/threading/sequenced_worker_pool.cc
index 7bbca92a2fb22d4bde9a50185a045faac004dea8..0e05f41d8008d71d85848ef8e6bbb96de924cd8f 100644
--- a/base/threading/sequenced_worker_pool.cc
+++ b/base/threading/sequenced_worker_pool.cc
@@ -619,6 +619,10 @@ bool SequencedWorkerPool::Inner::PostTask(
     // The trace_id is used for identifying the task in about:tracing.
     sequenced.trace_id = trace_id_++;
 
+    TRACE_EVENT_WITH_FLOW0(
+        "toplevel", "SequencedWorkerPool::Inner::PostTask",
+        TRACE_ID_MANGLE(GetTaskTraceID(sequenced, static_cast<void*>(this))),
+        TRACE_EVENT_FLAG_FLOW_OUT);
     TRACE_EVENT_FLOW_BEGIN0(TRACE_DISABLED_BY_DEFAULT("toplevel.flow"),
         "SequencedWorkerPool::PostTask",
         TRACE_ID_MANGLE(GetTaskTraceID(sequenced, static_cast<void*>(this))));
@@ -754,12 +758,14 @@ void SequencedWorkerPool::Inner::ThreadLoop(Worker* this_worker) {
       GetWorkStatus status =
           GetWork(&task, &wait_time, &delete_these_outside_lock);
       if (status == GET_WORK_FOUND) {
+        TRACE_EVENT_WITH_FLOW2(
+            "toplevel", "SequencedWorkerPool::ThreadLoop",
+            TRACE_ID_MANGLE(GetTaskTraceID(task, static_cast<void*>(this))),
+            TRACE_EVENT_FLAG_FLOW_IN, "src_file", task.posted_from.file_name(),
+            "src_func", task.posted_from.function_name());
         TRACE_EVENT_FLOW_END0(TRACE_DISABLED_BY_DEFAULT("toplevel.flow"),
             "SequencedWorkerPool::PostTask",
             TRACE_ID_MANGLE(GetTaskTraceID(task, static_cast<void*>(this))));
-        TRACE_EVENT2("toplevel", "SequencedWorkerPool::ThreadLoop",
-                     "src_file", task.posted_from.file_name(),
-                     "src_func", task.posted_from.function_name());
         int new_thread_id = WillRunWorkerTask(task);
         {
           AutoUnlock unlock(lock_);