[Offline Pages] Add a GCMAppHandler for offline page prefetch.

components/offline_pages/core/prefetch/prefetch_service.h

Index: components/offline_pages/core/prefetch/prefetch_service.h
diff --git a/components/offline_pages/core/prefetch/prefetch_service.h b/components/offline_pages/core/prefetch/prefetch_service.h
index 057d9f03a9cb764d8a215a249df8d76af9d06949..657b8ffa204b4eea4b63654af6afc3202f8ab1f3 100644
--- a/components/offline_pages/core/prefetch/prefetch_service.h
+++ b/components/offline_pages/core/prefetch/prefetch_service.h
@@ -10,6 +10,10 @@
 
 class GURL;
 
+namespace ntp_snippets {
+class ContentSuggestionsService;
+}
+
 namespace offline_pages {
 
 // Main class for the Offline Pages Prefetching feature, which will ask a server
@@ -42,6 +46,11 @@ class PrefetchService : public KeyedService {
 
   ~PrefetchService() override = default;
 
+  // Called at construction of the ContentSuggestionsService to begin observing
+  // events related to incoming articles.
+  virtual void ObserveContentSuggestionsService(
+      ntp_snippets::ContentSuggestionsService* service) = 0;
+
   // Called when a consumer has candidate URLs for the system to prefetch.
   // Duplicates are accepted by the PrefetchService but ignored.
   virtual void AddCandidatePrefetchURLs(