Add support for GCM subtypes to desktop Instance ID implementation

ios/chrome/browser/services/gcm/ios_chrome_gcm_profile_service_factory.h

Index: ios/chrome/browser/services/gcm/ios_chrome_gcm_profile_service_factory.h
diff --git a/ios/chrome/browser/services/gcm/ios_chrome_gcm_profile_service_factory.h b/ios/chrome/browser/services/gcm/ios_chrome_gcm_profile_service_factory.h
index edef82911fcab2d6e21d5ec27a5332016931900c..585abf4f14391dc2790f91f66672bf82350c7b01 100644
--- a/ios/chrome/browser/services/gcm/ios_chrome_gcm_profile_service_factory.h
+++ b/ios/chrome/browser/services/gcm/ios_chrome_gcm_profile_service_factory.h
@@ -6,6 +6,7 @@
 #define IOS_CHROME_BROWSER_SERVICES_GCM_IOS_CHROME_GCM_PROFILE_SERVICE_FACTORY_H_
 
 #include <memory>
+#include <string>
 
 #include "base/macros.h"
 #include "components/keyed_service/ios/browser_state_keyed_service_factory.h"
@@ -33,6 +34,13 @@ class IOSChromeGCMProfileServiceFactory
 
   static IOSChromeGCMProfileServiceFactory* GetInstance();
 
+  // Returns a string like "com.chrome.stable.ios" that should be used as the
+  // GCM category when an app_id is sent as a subtype instead of as a category.
+  // This string must never change during the lifetime of a Chrome install,
+  // since e.g. to unregister an Instance ID token the same category must be
+  // passed to GCM as was originally passed when registering it.
+  static std::string GetProductCategoryForSubtypes();
+
  private:
   friend struct base::DefaultSingletonTraits<IOSChromeGCMProfileServiceFactory>;