Make flow event traces disabled-by-default, and rename "ipc" and "task" categories to "toplevel"

base/threading/worker_pool_posix.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/threading/worker_pool_posix.h"
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/debug/trace_event.h"
 #include "base/lazy_instance.h"
@@ skipping 73 matching lines @@
   g_worker_pool_running_on_this_thread.Get().Set(true);
   const std::string name = base::StringPrintf(
       "%s/%d", name_prefix_.c_str(), PlatformThread::CurrentId());
   // Note |name.c_str()| must remain valid for for the whole life of the thread.
   PlatformThread::SetName(name.c_str());
 
   for (;;) {
     PendingTask pending_task = pool_->WaitForTask();
     if (pending_task.task.is_null())
       break;
-    TRACE_EVENT2("task", "WorkerThread::ThreadMain::Run",
+    TRACE_EVENT2("toplevel", "WorkerThread::ThreadMain::Run",
         "src_file", pending_task.posted_from.file_name(),
         "src_func", pending_task.posted_from.function_name());
 
     TrackedTime start_time =
         tracked_objects::ThreadData::NowForStartOfRun(pending_task.birth_tally);
 
     pending_task.task.Run();
 
     tracked_objects::ThreadData::TallyRunOnWorkerThreadIfTracking(
         pending_task.birth_tally, TrackedTime(pending_task.time_posted),
@@ skipping 88 matching lines @@
       return PendingTask(FROM_HERE, base::Closure());
     }
   }
 
   PendingTask pending_task = pending_tasks_.front();
   pending_tasks_.pop();
   return pending_task;
 }
 
 }  // namespace base