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

extensions/browser/file_reader.cc

 // 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.
 
 #include "extensions/browser/file_reader.h"
 
 #include "base/bind.h"
 #include "base/files/file_util.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "content/public/browser/browser_thread.h"
 
 using content::BrowserThread;
 
-FileReader::FileReader(const extensions::ExtensionResource& resource,
-                       const Callback& callback)
+FileReader::FileReader(
+    const extensions::ExtensionResource& resource,
+    const OptionalFileThreadTaskCallback& optional_file_thread_task_callback,
+    const DoneCallback& done_callback)
     : resource_(resource),
-      callback_(callback),
+      optional_file_thread_task_callback_(optional_file_thread_task_callback),
+      done_callback_(done_callback),
       origin_task_runner_(base::ThreadTaskRunnerHandle::Get()) {}
 
 void FileReader::Start() {
   BrowserThread::PostTask(
       BrowserThread::FILE, FROM_HERE,
       base::Bind(&FileReader::ReadFileOnBackgroundThread, this));
 }
 
 FileReader::~FileReader() {}
 
 void FileReader::ReadFileOnBackgroundThread() {
   std::unique_ptr<std::string> data(new std::string());
   bool success = base::ReadFileToString(resource_.GetFilePath(), data.get());
+
+  if (!optional_file_thread_task_callback_.is_null())
+    optional_file_thread_task_callback_.Run(success, data.get());
+
   origin_task_runner_->PostTask(
-      FROM_HERE, base::Bind(callback_, success, base::Passed(std::move(data))));
+      FROM_HERE,
+      base::Bind(done_callback_, success, base::Passed(std::move(data))));
 }