Use FILE thread for disk operations in TraceSubscriberStdio

base/test/trace_event_analyzer_unittest.cc

Index: base/test/trace_event_analyzer_unittest.cc
===================================================================
--- base/test/trace_event_analyzer_unittest.cc	(revision 123084)
+++ base/test/trace_event_analyzer_unittest.cc	(working copy)
@@ -15,8 +15,7 @@
  public:
   void ManualSetUp();
   void OnTraceDataCollected(
-      const scoped_refptr<base::debug::TraceLog::RefCountedString>&
-          json_events_str);
+      const scoped_refptr<base::RefCountedString>& json_events_str);
   void BeginTracing();
   void EndTracing();
 
@@ -36,9 +35,8 @@
 }
 
 void TraceEventAnalyzerTest::OnTraceDataCollected(
-    const scoped_refptr<base::debug::TraceLog::RefCountedString>&
-        json_events_str) {
-  buffer_.AddFragment(json_events_str->data);
+    const scoped_refptr<base::RefCountedString>& json_events_str) {
+  buffer_.AddFragment(json_events_str->data());
 }
 
 void TraceEventAnalyzerTest::BeginTracing() {