Use SequencedWorkerPool for disk operations in TraceSubscriberStdio.

chrome/browser/ui/webui/tracing_ui.cc

Index: chrome/browser/ui/webui/tracing_ui.cc
diff --git a/chrome/browser/ui/webui/tracing_ui.cc b/chrome/browser/ui/webui/tracing_ui.cc
index 3a1c492af7e7e69876f514706afcf733d779c634..9f5bc5fe1b76f33c8b297fde651eb217c39a6887 100644
--- a/chrome/browser/ui/webui/tracing_ui.cc
+++ b/chrome/browser/ui/webui/tracing_ui.cc
@@ -73,7 +73,8 @@ class TracingMessageHandler
 
   // TraceSubscriber implementation.
   virtual void OnEndTracingComplete();
-  virtual void OnTraceDataCollected(const std::string& trace_fragment);
+  virtual void OnTraceDataCollected(
+      const scoped_refptr<base::RefCountedString>& trace_fragment);
   virtual void OnTraceBufferPercentFullReply(float percent_full);
 
   // GpuDataManagerObserver implementation.
@@ -379,7 +380,7 @@ void TracingMessageHandler::OnEndTracingComplete() {
 }
 
 void TracingMessageHandler::OnTraceDataCollected(
-    const std::string& trace_fragment) {
+    const scoped_refptr<base::RefCountedString>& trace_fragment) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 
   base::debug::TraceResultBuffer::SimpleOutput output;
@@ -387,7 +388,7 @@ void TracingMessageHandler::OnTraceDataCollected(
   trace_buffer.SetOutputCallback(output.GetCallback());
   output.Append("tracingController.onTraceDataCollected(");
   trace_buffer.Start();
-  trace_buffer.AddFragment(trace_fragment);
+  trace_buffer.AddFragment(trace_fragment->data());
   trace_buffer.Finish();
   output.Append(");");