Make FileReader return ownership of the string content

extensions/browser/api/execute_code_function.h

Index: extensions/browser/api/execute_code_function.h
diff --git a/extensions/browser/api/execute_code_function.h b/extensions/browser/api/execute_code_function.h
index a483e13ab3b57379e630d2d78efd4ffc311cc8da..3ba96e21317feb317c80eedc95e299434b9f52dd 100644
--- a/extensions/browser/api/execute_code_function.h
+++ b/extensions/browser/api/execute_code_function.h
@@ -43,7 +43,7 @@ class ExecuteCodeFunction : public AsyncExtensionFunction {
   // Called when contents from the loaded file have been localized.
   void DidLoadAndLocalizeFile(const std::string& file,
                               bool success,
-                              const std::string& data);
+                              std::unique_ptr<std::string> data);
 
   const HostID& host_id() const { return host_id_; }
   void set_host_id(const HostID& host_id) { host_id_ = host_id; }
@@ -54,12 +54,12 @@ class ExecuteCodeFunction : public AsyncExtensionFunction {
  private:
   // Called when contents from the file whose path is specified in JSON
   // arguments has been loaded.
-  void DidLoadFile(bool success, const std::string& data);
+  void DidLoadFile(bool success, std::unique_ptr<std::string> data);
 
   // Runs on FILE thread. Loads message bundles for the extension and
   // localizes the CSS data. Calls back DidLoadAndLocalizeFile on the UI thread.
   void GetFileURLAndLocalizeCSS(ScriptExecutor::ScriptType script_type,
-                                const std::string& data,
+                                std::unique_ptr<std::string> data,
                                 const std::string& extension_id,
                                 const base::FilePath& extension_path,
                                 const std::string& extension_default_locale);