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

chrome/browser/chromeos/accessibility/accessibility_manager.cc

Index: chrome/browser/chromeos/accessibility/accessibility_manager.cc
diff --git a/chrome/browser/chromeos/accessibility/accessibility_manager.cc b/chrome/browser/chromeos/accessibility/accessibility_manager.cc
index ac240ea981243e5a32b6a65a4f291e84e50fdb06..24a12771f259b2bd49644ebeb34ad99f7ce5663f 100644
--- a/chrome/browser/chromeos/accessibility/accessibility_manager.cc
+++ b/chrome/browser/chromeos/accessibility/accessibility_manager.cc
@@ -185,8 +185,11 @@ class ContentScriptLoader {
 
     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();
   }