Use SequencedWorkerPool for disk operations in TraceSubscriberStdio.

base/debug/trace_event_impl.h

Index: base/debug/trace_event_impl.h
diff --git a/base/debug/trace_event_impl.h b/base/debug/trace_event_impl.h
index 1806f8939fbf7756f7ed46c9956ceab32bc1181d..04aea188239b7ce8f193f6e5ead09ff41a2fe5bf 100644
--- a/base/debug/trace_event_impl.h
+++ b/base/debug/trace_event_impl.h
@@ -42,8 +42,6 @@ struct StaticMemorySingletonTraits;
 
 namespace base {
 
-class RefCountedString;
-
 namespace debug {
 
 const int kTraceMaxNumArgs = 2;
@@ -221,8 +219,7 @@ class BASE_EXPORT TraceLog {
   // silently dropped. The callback must be thread safe. The string format is
   // undefined. Use TraceResultBuffer to convert one or more trace strings to
   // JSON.
-  typedef RefCountedData<std::string> RefCountedString;
-  typedef base::Callback<void(const scoped_refptr<RefCountedString>&)>
+  typedef base::Callback<void(const scoped_refptr<base::RefCountedString>&)>
       OutputCallback;
   void SetOutputCallback(const OutputCallback& cb);