Remove some UI->FILE->UI thread hops in ExecuteCodeFunction

extensions/browser/file_reader.h

 // Copyright (c) 2011 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.
 
 #ifndef EXTENSIONS_BROWSER_FILE_READER_H_
 #define EXTENSIONS_BROWSER_FILE_READER_H_
 
 #include <string>
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
 #include "extensions/common/extension_resource.h"
 
 // This file defines an interface for reading a file asynchronously on a
 // background thread.
 // Consider abstracting out a FilePathProvider (ExtensionResource) and moving
 // back to chrome/browser/net if other subsystems want to use it.
 class FileReader : public base::RefCountedThreadSafe<FileReader> {
  public:
   // Reports success or failure and the data of the file upon success.
-  typedef base::Callback<void(bool, std::unique_ptr<std::string>)> Callback;
+  using DoneCallback = base::Callback<void(bool, std::unique_ptr<std::string>)>;
+  // Lets the caller accomplish tasks on the file data, after the file content
+  // has been read.
+  // If the file reading doesn't succeed, this will be ignored.
+  using OptionalFileThreadTaskCallback = base::Callback<void(std::string*)>;
 
   FileReader(const extensions::ExtensionResource& resource,
-             const Callback& callback);
+             const OptionalFileThreadTaskCallback& file_thread_task_callback,
+             const DoneCallback& done_callback);
 
   // Called to start reading the file on a background thread.  Upon completion,
   // the callback will be notified of the results.
   void Start();
 
  private:
   friend class base::RefCountedThreadSafe<FileReader>;
 
   virtual ~FileReader();
 
   void ReadFileOnBackgroundThread();
 
   extensions::ExtensionResource resource_;
-  Callback callback_;
+  OptionalFileThreadTaskCallback optional_file_thread_task_callback_;
+  DoneCallback done_callback_;
   const scoped_refptr<base::SingleThreadTaskRunner> origin_task_runner_;
 };
 
 #endif  // EXTENSIONS_BROWSER_FILE_READER_H_