Add a unittest for image_data_fetcher

components/image_fetcher/image_data_fetcher.h

Index: components/image_fetcher/image_data_fetcher.h
diff --git a/components/image_fetcher/image_data_fetcher.h b/components/image_fetcher/image_data_fetcher.h
index bd476ef09f0824fa3076a76430805ef6c9571857..fc1188d4e7d04256e36002dd0ee5003d7180161d 100644
--- a/components/image_fetcher/image_data_fetcher.h
+++ b/components/image_fetcher/image_data_fetcher.h
@@ -49,6 +49,13 @@ class ImageDataFetcher : public net::URLFetcherDelegate {
 
   scoped_refptr<net::URLRequestContextGetter> url_request_context_getter_;
 
+  // The next id to use for a newly created URLFetcher. Each URLFetcher gets an

[Bernhard Bauer, 2016/06/20 13:36:15]

Nit: I would capitalize "ID" (you're not talking about
https://en.wikipedia.org/wiki/Id,_ego_and_super-ego#Id 😉).

[markusheintz_, 2016/06/20 14:30:17]

Done.

+  // id when it id created. The |url_fetcher_id_| is incremeted by one for each

[Bernhard Bauer, 2016/06/20 13:36:15]

"[...] when it is created". Also, "incremented".

[markusheintz_, 2016/06/20 14:30:16]

Done.

+  // 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 url_fetcher_id_;

[Marc Treib, 2016/06/20 14:47:29]

next_url_fetcher_id_ ?

[markusheintz_, 2016/06/20 15:06:52]

Done.

+
   DISALLOW_COPY_AND_ASSIGN(ImageDataFetcher);
 };