Use SequencedWorkerPool for disk operations in TraceSubscriberStdio.

chrome/browser/automation/automation_provider.cc

Index: chrome/browser/automation/automation_provider.cc
diff --git a/chrome/browser/automation/automation_provider.cc b/chrome/browser/automation/automation_provider.cc
index b4a32cf70b7051cd41f10a047df7e5692c5b32ca..38b1b0b5a3867d1b763bceb6bf3d1c89a8ce63f4 100644
--- a/chrome/browser/automation/automation_provider.cc
+++ b/chrome/browser/automation/automation_provider.cc
@@ -406,8 +406,8 @@ void AutomationProvider::OnEndTracingComplete() {
 }
 
 void AutomationProvider::OnTraceDataCollected(
-    const std::string& trace_fragment) {
-  tracing_data_.trace_output.push_back(trace_fragment);
+    const scoped_refptr<base::RefCountedString>& trace_fragment) {
+  tracing_data_.trace_output.push_back(trace_fragment->data());
 }
 
 bool AutomationProvider::OnMessageReceived(const IPC::Message& message) {