Use FILE thread for disk operations in TraceSubscriberStdio

content/browser/trace_subscriber_stdio.h

Index: content/browser/trace_subscriber_stdio.h
===================================================================
--- content/browser/trace_subscriber_stdio.h	(revision 123084)
+++ content/browser/trace_subscriber_stdio.h	(working copy)
@@ -12,32 +12,26 @@
 #include "content/browser/trace_controller.h"
 #include "content/common/content_export.h"
 
+namespace content {
+
+class TraceSubscriberStdioImpl;
+
 // Stdio implementation of TraceSubscriber. Use this to write traces to a file.
 class CONTENT_EXPORT TraceSubscriberStdio : public TraceSubscriber {
  public:
-  TraceSubscriberStdio();
   // Creates or overwrites the specified file. Check IsValid() for success.
   explicit TraceSubscriberStdio(const FilePath& path);
+  virtual ~TraceSubscriberStdio();
 
-  // Creates or overwrites the specified file. Returns true on success.
-  bool OpenFile(const FilePath& path);
-  // Finishes json output and closes file.
-  void CloseFile();
-
-  // Returns TRUE if we're currently writing data to a file.
-  bool IsValid();
-
   // Implementation of TraceSubscriber
   virtual void OnEndTracingComplete() OVERRIDE;
-  virtual void OnTraceDataCollected(const std::string& trace_fragment) OVERRIDE;
+  virtual void OnTraceDataCollected(
+      const scoped_refptr<base::RefCountedString>& data_ptr) OVERRIDE;
 
-  virtual ~TraceSubscriberStdio();
-
  private:
-  void Write(const std::string& output_str);
-
-  FILE* file_;
-  base::debug::TraceResultBuffer trace_buffer_;
+  scoped_refptr<TraceSubscriberStdioImpl> impl_;
 };
 
+}  // namespace content
+
 #endif  // CONTENT_BROWSER_TRACE_SUBSCRIBER_STDIO_H_