Add support for GCM subtypes to desktop Instance ID implementation

components/gcm_driver/registration_info.h

Index: components/gcm_driver/registration_info.h
diff --git a/components/gcm_driver/registration_info.h b/components/gcm_driver/registration_info.h
index b9ba0a1b038c78f32ef11219796ef64f7eff4490..aef7ade0320ce702232dd3b5c26de9db5624e064 100644
--- a/components/gcm_driver/registration_info.h
+++ b/components/gcm_driver/registration_info.h
@@ -105,7 +105,8 @@ struct InstanceIDTokenInfo : public RegistrationInfo {
   std::string scope;
 
   // Allows including a small number of string key/value pairs that will be
-  // associated with the token and may be used in processing the request.
+  // associated with the token and may be used in processing the request. These
+  // are not serialized/deserialized.
   std::map<std::string, std::string> options;
 };