Use FILE thread for disk operations in TraceSubscriberStdio

chrome/browser/automation/automation_provider.h

 // 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.
 
 // This implements a browser-side endpoint for UI automation activity.
 // The client-side endpoint is implemented by AutomationProxy.
 // The entire lifetime of this object should be contained within that of
 // the BrowserProcess, and in particular the NotificationService that's
 // hung off of it.
 
@@ skipping 223 matching lines @@
 
  private:
   // Storage for EndTracing() to resume operations after a callback.
   struct TracingData {
     std::list<std::string> trace_output;
     scoped_ptr<IPC::Message> reply_message;
   };
 
   // TraceSubscriber:
   virtual void OnEndTracingComplete() OVERRIDE;
-  virtual void OnTraceDataCollected(const std::string& trace_fragment) OVERRIDE;
+  virtual void OnTraceDataCollected(
+      const scoped_refptr<base::RefCountedString>& trace_fragment) OVERRIDE;
 
   void OnUnhandledMessage(const IPC::Message& message);
 
   // Clear and reinitialize the automation IPC channel.
   bool ReinitializeChannel();
 
   // IPC Message callbacks.
   void WindowSimulateDrag(int handle,
                           const std::vector<gfx::Point>& drag_path,
                           int flags,
@@ skipping 166 matching lines @@
   std::string channel_id_;
 
   // Trace data that has been collected but not flushed to the automation
   // client.
   TracingData tracing_data_;
 
   DISALLOW_COPY_AND_ASSIGN(AutomationProvider);
 };
 
 #endif  // CHROME_BROWSER_AUTOMATION_AUTOMATION_PROVIDER_H_