[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 9e5ec6cbf624796bcf6e14499e9a367ded03c657..eebf5410665379a61801235de250817232310c6b 100644
--- a/components/offline_pages/core/prefetch/prefetch_service.h
+++ b/components/offline_pages/core/prefetch/prefetch_service.h
@@ -7,9 +7,14 @@
 
 #include "components/keyed_service/core/keyed_service.h"
 
+namespace ntp_snippets {
+class ContentSuggestionsService;
+}
+
 namespace offline_pages {
 
 class PrefetchDispatcher;
+class PrefetchGCMHandler;
 
 // Main class and entry point for the Offline Pages Prefetching feature, that
 // controls the lifetime of all major subcomponents of the prefetching system.
@@ -18,6 +23,16 @@ class PrefetchService : public KeyedService {
   ~PrefetchService() override = default;
 
   virtual PrefetchDispatcher* GetDispatcher() = 0;
+
+  // Called at construction of the ContentSuggestionsService to begin observing
+  // events related to incoming articles.
+  virtual void ObserveContentSuggestionsService(
+      ntp_snippets::ContentSuggestionsService* service) = 0;
+
+  // Called at construction of the GCMProfileService to register an app handler
+  // for incoming messages.
+  virtual void AddPrefetchGCMHandler(
+      std::unique_ptr<PrefetchGCMHandler> app_handler) = 0;
 };
 
 }  // namespace offline_pages