Add support for GCM subtypes to desktop Instance ID implementation

ios/chrome/browser/application_context_impl.cc

Index: ios/chrome/browser/application_context_impl.cc
diff --git a/ios/chrome/browser/application_context_impl.cc b/ios/chrome/browser/application_context_impl.cc
index 4f7ace27c42356a3cf80bf31b99e06da561a19dd..567118744663ea4b607061db25517decb5b97e3c 100644
--- a/ios/chrome/browser/application_context_impl.cc
+++ b/ios/chrome/browser/application_context_impl.cc
@@ -338,6 +338,19 @@ void ApplicationContextImpl::CreateGCMDriver() {
 
   base::FilePath store_path;
   CHECK(PathService::Get(ios::DIR_GLOBAL_GCM_STORE, &store_path));
+
+// These strings 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 FCM as was originally passed when registering it.
+#if defined(GOOGLE_CHROME_BUILD)
+  std::string channel = ::GetChannelString();
+  if (channel.empty())
+    channel = "stable";
+  std::string product_category_for_subtypes = "com.chrome." + channel + ".ios";
+#else
+  std::string product_category_for_subtypes = "org.chromium.unknown.ios";
+#endif
+
   base::SequencedWorkerPool* worker_pool = web::WebThread::GetBlockingPool();
   scoped_refptr<base::SequencedTaskRunner> blocking_task_runner(
       worker_pool->GetSequencedTaskRunnerWithShutdownBehavior(
@@ -347,6 +360,7 @@ void ApplicationContextImpl::CreateGCMDriver() {
   gcm_driver_ = gcm::CreateGCMDriverDesktop(
       base::WrapUnique(new gcm::GCMClientFactory), GetLocalState(), store_path,
       GetSystemURLRequestContext(), ::GetChannel(),
+      product_category_for_subtypes,
       web::WebThread::GetTaskRunnerForThread(web::WebThread::UI),
       web::WebThread::GetTaskRunnerForThread(web::WebThread::IO),
       blocking_task_runner);