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

chrome/browser/chromeos/accessibility/accessibility_extension_loader.cc

 // Copyright 2016 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 "chrome/browser/chromeos/accessibility/accessibility_extension_loader.h"
 
 #include "base/callback.h"
 #include "base/callback_helpers.h"
 #include "base/path_service.h"
 #include "chrome/browser/chromeos/login/lock/screen_locker.h"
@@ skipping 72 matching lines @@
   // Finally, call this method once to fetch all of the resources and
   // load them. This method will delete this object when done.
   void Run() {
     if (resources_.empty()) {
       delete this;
       return;
     }
 
     extensions::ExtensionResource resource = resources_.front();
     resources_.pop();
-    scoped_refptr<FileReader> reader(
-        new FileReader(resource, base::Bind(&ContentScriptLoader::OnFileLoaded,
-                                            base::Unretained(this))));
+    scoped_refptr<FileReader> reader(new FileReader(
+        resource,
+        FileReader::OptionalFileThreadTaskCallback(),  // null callback.
+        base::Bind(&ContentScriptLoader::OnFileLoaded,
+                   base::Unretained(this))));
     reader->Start();
   }
 
  private:
   void OnFileLoaded(bool success, std::unique_ptr<std::string> data) {
     if (success) {
       content::RenderViewHost* render_view_host =
           content::RenderViewHost::FromID(render_process_id_, render_view_id_);
       if (render_view_host)
         ExecuteScriptHelper(render_view_host, *data, extension_id_);
@@ skipping 194 matching lines @@
         weak_ptr_factory_.GetWeakPtr(), extension_service,
         render_view_host->GetProcess()->GetID(),
         render_view_host->GetRoutingID(), done_cb);
   }
 
   extension_service->component_loader()->AddComponentFromDir(
       extension_path_, extension_id_.c_str(), done_cb);
 }
 
 }  // namespace chromeos