FetchMore functionality backend

components/ntp_snippets/remote/ntp_snippets_fetcher.h

Index: components/ntp_snippets/remote/ntp_snippets_fetcher.h
diff --git a/components/ntp_snippets/remote/ntp_snippets_fetcher.h b/components/ntp_snippets/remote/ntp_snippets_fetcher.h
index f5bbf27412cc229053c8a01e15988a324e07afc0..fc727b2d01cf49bf6fee1cbb60abbc064b6590e3 100644
--- a/components/ntp_snippets/remote/ntp_snippets_fetcher.h
+++ b/components/ntp_snippets/remote/ntp_snippets_fetcher.h
@@ -65,7 +65,7 @@ class NTPSnippetsFetcher : public OAuth2TokenService::Consumer,
   // occur, |snippets| contains no value (no actual vector in base::Optional).
   // Error details can be retrieved using last_status().
   using SnippetsAvailableCallback =
-      base::Callback<void(OptionalFetchedCategories fetched_categories)>;
+      base::OnceCallback<void(OptionalFetchedCategories fetched_categories)>;

[Marc Treib, 2016/10/18 08:17:45]

If the callback is now for one fetch only, it should be passed directly to
FetchSnippets - no point in a separate SetCallback.

One thing to consider: What happends when FetchSnippets is called again before
the previous one has completed?

[fhorschig, 2016/10/20 13:10:38]

The callback is passed into FetchSnippets.

The descriptive comment of this function states: 
"If an ongoing fetch exists, it will be cancelled and a new one without
triggering an additional callback [...]"
I haven't verified this behavior but I have faith in whoever wrote this.

[Marc Treib, 2016/10/20 16:51:39]

Well, that's what the previous behavior amounted to, but you're changing that.
See other comment.

[tschumann, 2016/10/24 06:38:25]

never trust untested code ;-) A test would be really nice for that scenario.

[Marc Treib, 2016/10/28 14:49:48]

We decided to fix this after this CL (crbug.com/659931).

 
   // Enumeration listing all possible outcomes for fetch attempts. Used for UMA
   // histograms, so do not change existing values. Insert new values at the end,
@@ -104,7 +104,7 @@ class NTPSnippetsFetcher : public OAuth2TokenService::Consumer,
 
   // Set a callback that is called when a new set of snippets are downloaded,
   // overriding any previously set callback.
-  void SetCallback(const SnippetsAvailableCallback& callback);
+  void SetCallback(SnippetsAvailableCallback callback);
 
   // Fetches snippets from the server. |hosts| restricts the results to a set of
   // hosts, e.g. "www.google.com". If |hosts| is empty, no host restrictions are