Add support for GCM subtypes to desktop Instance ID implementation

google_apis/gcm/engine/registration_request.h

Index: google_apis/gcm/engine/registration_request.h
diff --git a/google_apis/gcm/engine/registration_request.h b/google_apis/gcm/engine/registration_request.h
index cb45c974a6664372c4bfe581ba8ed1b7dc67212e..2395e4975cc4ea131bfdf73bf2352bd5264c3ddd 100644
--- a/google_apis/gcm/engine/registration_request.h
+++ b/google_apis/gcm/engine/registration_request.h
@@ -67,15 +67,24 @@ class GCM_EXPORT RegistrationRequest : public net::URLFetcherDelegate {
   struct GCM_EXPORT RequestInfo {
     RequestInfo(uint64_t android_id,
                 uint64_t security_token,
-                const std::string& app_id);
+                const std::string& category,
+                const std::string& subtype);
     ~RequestInfo();
 
     // Android ID of the device.
     uint64_t android_id;
     // Security token of the device.
     uint64_t security_token;
-    // Application ID.
-    std::string app_id;
+
+    // Application ID used in Chrome to refer to registration/token's owner.
+    const std::string& app_id() const {
+      return subtype.empty() ? category : subtype;
+    }
+
+    // GCM category field.
+    std::string category;
+    // GCM subtype field.
+    std::string subtype;
   };
 
   // Encapsulates the custom logic that is needed to build and process the