Migrate comment in BackgroundTracingManager off of FILE thread.

Bug: 689520
Change-Id: I2bd06d69cb54fc81edb8a462caa6458effb5bdfe
Reviewed-on: https://chromium-review.googlesource.com/580397Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488658}

content/public/browser/background_tracing_manager.h

@@ -22,30 +22,26 @@ class BackgroundTracingManager {
  public:
   CONTENT_EXPORT static BackgroundTracingManager* GetInstance();
 
-  // ReceiveCallback will will be called on the UI thread every time the
-  // BackgroundTracingManager finalizes a trace. The first parameter of
-  // this callback is the trace data. The second is metadata that was
-  // generated and embedded into the trace. The third is a callback to
-  // notify the BackgroundTracingManager that you've finished processing
-  // the trace data.
+  // ReceiveCallback will be called on the UI thread every time the
+  // BackgroundTracingManager finalizes a trace. The first parameter of this
+  // callback is the trace data. The second is metadata that was generated and
+  // embedded into the trace. The third is a callback to notify the
+  // BackgroundTracingManager that you've finished processing the trace data.
   //
   // Example:
   //
   // void Upload(const scoped_refptr<base::RefCountedString>& data,
   //             std::unique_ptr<base::DictionaryValue>,
   //             base::Closure done_callback) {
-  //   BrowserThread::PostTaskAndReply(
-  //       BrowserThread::FILE,
-  //       FROM_HERE,
-  //       base::Bind(&DoUploadOnFileThread, data),
-  //       done_callback
-  //     );
+  //   base::PostTaskWithTraitsAndReply(
+  //       FROM_HERE, {base::MayBlock(), base::TaskPriority::BACKGROUND},
+  //       base::Bind(&DoUploadInBackground, data), done_callback);
   // }
   //
-  typedef base::Callback<void(const scoped_refptr<base::RefCountedString>&,
-                              std::unique_ptr<const base::DictionaryValue>,
-                              base::Closure)>
-      ReceiveCallback;
+  using ReceiveCallback =
+      base::Callback<void(const scoped_refptr<base::RefCountedString>&,
+                          std::unique_ptr<const base::DictionaryValue>,
+                          base::Closure)>;
 
   // Set the triggering rules for when to start recording.
   //