Make FileReader return ownership of the string content

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 10a65ff6b8cff198e1903a69b1546caae16375e6..9b391f8635be496d1792bfa6d52f4307aec29bc3 100644
--- a/extensions/browser/file_reader.h
+++ b/extensions/browser/file_reader.h
@@ -19,7 +19,7 @@
 class FileReader : public base::RefCountedThreadSafe<FileReader> {
  public:
   // Reports success or failure and the data of the file upon success.
-  typedef base::Callback<void(bool, const std::string&)> Callback;
+  typedef base::Callback<void(bool, std::unique_ptr<std::string>)> Callback;
 
   FileReader(const extensions::ExtensionResource& resource,
              const Callback& callback);