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

components/offline_pages/content/suggested_articles_observer_unittest.cc

Index: components/offline_pages/content/suggested_articles_observer_unittest.cc
diff --git a/components/offline_pages/content/suggested_articles_observer_unittest.cc b/components/offline_pages/content/suggested_articles_observer_unittest.cc
index 2a988ddf3673abb16806b75846de6321a0650e82..892262e95cdb3ecf81898b1317437d482ed94865 100644
--- a/components/offline_pages/content/suggested_articles_observer_unittest.cc
+++ b/components/offline_pages/content/suggested_articles_observer_unittest.cc
@@ -16,6 +16,10 @@
 using ntp_snippets::Category;
 using ntp_snippets::ContentSuggestion;
 
+namespace gcm {
+class GCMDriver;
+}
+
 namespace offline_pages {
 
 namespace {
@@ -52,6 +56,7 @@ class TestingPrefetchService : public PrefetchService {
   void BeginBackgroundTask(
       std::unique_ptr<ScopedBackgroundTask> task) override {}
   void StopBackgroundTask(ScopedBackgroundTask* task) override {}
+  void RegisterWithGCMDriver(gcm::GCMDriver* driver) override {}
 
   std::vector<PrefetchURL> latest_prefetch_urls;
   std::unique_ptr<ClientId> last_removed_client_id;