Use FILE thread for disk operations in TraceSubscriberStdio

content/browser/trace_controller.h

Index: content/browser/trace_controller.h
===================================================================
--- content/browser/trace_controller.h	(revision 123084)
+++ content/browser/trace_controller.h	(working copy)
@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "base/debug/trace_event.h"
+#include "base/memory/ref_counted_memory.h"
 #include "base/memory/singleton.h"
 #include "content/common/content_export.h"
 
@@ -26,7 +27,8 @@
   // Called 0 or more times between TraceController::BeginTracing and
   // OnEndTracingComplete. Use base::debug::TraceResultBuffer to convert one or
   // more trace fragments to JSON.
-  virtual void OnTraceDataCollected(const std::string& trace_fragment) = 0;
+  virtual void OnTraceDataCollected(
+      const scoped_refptr<base::RefCountedString>& trace_fragment) = 0;
   // Called once after TraceController::GetKnownCategoriesAsync.
   virtual void OnKnownCategoriesCollected(
       const std::set<std::string>& known_categories);
@@ -154,8 +156,7 @@
   void OnTracingBegan(TraceSubscriber* subscriber);
   void OnEndTracingAck(const std::vector<std::string>& known_categories);
   void OnTraceDataCollected(
-      const scoped_refptr<base::debug::TraceLog::RefCountedString>&
-          events_str_ptr);
+      const scoped_refptr<base::RefCountedString>& events_str_ptr);
   void OnTraceBufferFull();
   void OnTraceBufferPercentFullReply(float percent_full);