Add a unittest for image_data_fetcher

components/image_fetcher/image_data_fetcher.h

 // 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.
 
 #ifndef COMPONENTS_IMAGE_FETCHER_IMAGE_DATA_FETCHER_H_
 #define COMPONENTS_IMAGE_FETCHER_IMAGE_DATA_FETCHER_H_
 
 #include <map>
 #include <memory>
 #include <string>
@@ skipping 31 matching lines @@
 
   // Method inherited from URLFetcherDelegate
   void OnURLFetchComplete(const net::URLFetcher* source) override;
 
   // All active image url requests.
   std::map<const net::URLFetcher*, std::unique_ptr<ImageDataFetcherRequest>>
       pending_requests_;
 
   scoped_refptr<net::URLRequestContextGetter> url_request_context_getter_;
 
+  // The next ID to use for a newly created URLFetcher. Each URLFetcher gets an
+  // id when it is created. The |url_fetcher_id_| is incremented by one for each
+  // newly created URLFetcher. The URLFetcher ID can be used during testing to
+  // get individual URLFetchers and modify their state. Outside of tests this ID
+  // is not used.
+  int next_url_fetcher_id_;
+
   DISALLOW_COPY_AND_ASSIGN(ImageDataFetcher);
 };
 
 }  // namespace image_fetcher
 
 #endif  // COMPONENTS_IMAGE_FETCHER_IMAGE_DATA_FETCHER_H_