Support action button to fetch more content suggestions

components/ntp_snippets/content_suggestions_provider.h

Index: components/ntp_snippets/content_suggestions_provider.h
diff --git a/components/ntp_snippets/content_suggestions_provider.h b/components/ntp_snippets/content_suggestions_provider.h
index 2f343a47dc4bf918334e9eea8d58026f078bf55b..99f2e5de6a8646819852006ce1fdecb9191bc9a4 100644
--- a/components/ntp_snippets/content_suggestions_provider.h
+++ b/components/ntp_snippets/content_suggestions_provider.h
@@ -81,6 +81,12 @@ class ContentSuggestionsProvider {
   // Returns the meta information for the given |category|.
   virtual CategoryInfo GetCategoryInfo(Category category) = 0;
 
+  // Triggers a fetch for new suggestions (over the network, for example) for
+  // the given |category|. This must be initiated by the user (e.g. by clicking
+  // the a "More" button). New suggestions will become available through

[Marc Treib, 2016/08/10 09:40:31]

nit: remove "a"

[Philipp Keck, 2016/08/10 13:14:46]

Done.

+  // |Observer::OnNewSuggestions|.
+  virtual void FetchMoreSuggestions(Category category) = 0;

[Marc Treib, 2016/08/10 09:40:31]

Hm. For now, we don't expect this method to ever actually get called, right? All
the "more" buttons we'll have in M54 will have special UI actions. Might have
been better to leave this out until we actually need it.
But since you've already written it, and we're pretty sure we are actually gonna
need it, I guess might as well keep it now.

[Philipp Keck, 2016/08/10 13:14:46]

Acknowledged.

We could, instead, put sth. like a notreached in the else-branch here:
https://codereview.chromium.org/2232783002/diff/1/chrome/android/java/src/org...

But actually, I think we will need this: Our status cards will become
per-section (or are already). We will have a "All Read --> Reload" card under
the "Articles for you" header. In M55, this will be replaced with a "All Read,
nothing here" card and the usual "More" button under the status-empty-card. But
for M54, it's the Reload-button on the card. And instead of calling the static
FetchSnippets across the bridge, we might as well trigger the section-specific
FetchMoreSuggestion as it's supposed to be. (Although they end up in the same
place, effectively).

+
   // Dismisses the suggestion with the given ID. A provider needs to ensure that
   // a once-dismissed suggestion is never delivered again (through the
   // Observer). The provider must not call Observer::OnSuggestionsChanged if the