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

extensions/browser/file_reader.h

Index: extensions/browser/file_reader.h
diff --git a/extensions/browser/file_reader.h b/extensions/browser/file_reader.h
index 9b391f8635be496d1792bfa6d52f4307aec29bc3..a9f88688cca1e136125037ffdb312357cb91172e 100644
--- a/extensions/browser/file_reader.h
+++ b/extensions/browser/file_reader.h
@@ -19,10 +19,15 @@
 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.
@@ -36,7 +41,8 @@ class FileReader : public base::RefCountedThreadSafe<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_;
 };