Add support for GCM subtypes to desktop Instance ID implementation

components/gcm_driver/fake_gcm_client.h

Index: components/gcm_driver/fake_gcm_client.h
diff --git a/components/gcm_driver/fake_gcm_client.h b/components/gcm_driver/fake_gcm_client.h
index 4ba11140e74b9fbcdf3491723be9571ee4cd1859..e7631f0add20f62e30cab29251e7a8f4ee7ba4fa 100644
--- a/components/gcm_driver/fake_gcm_client.h
+++ b/components/gcm_driver/fake_gcm_client.h
@@ -113,6 +113,7 @@ class FakeGCMClient : public GCMClient {
                            const std::string& message_id);
 
   Delegate* delegate_;
+  std::string category_for_subtypes_;
   bool started_;
   StartMode start_mode_;
   StartModeOverridding start_mode_overridding_;