Add command line overrides for GCM checkin URL & MCS endpoint

components/gcm_driver/gcm_client_impl.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/gcm_driver/gcm_client_impl.h"
 
 #include "base/bind.h"
 #include "base/files/file_path.h"
 #include "base/location.h"
 #include "base/logging.h"
@@ skipping 460 matching lines @@
   }
 
   state_ = INITIAL_DEVICE_CHECKIN;
   device_checkin_info_.Reset();
   StartCheckin();
 }
 
 void GCMClientImpl::InitializeMCSClient() {
   std::vector<GURL> endpoints;
   endpoints.push_back(gservices_settings_.GetMCSMainEndpoint());
-  endpoints.push_back(gservices_settings_.GetMCSFallbackEndpoint());
+  GURL fallback_endpoint = gservices_settings_.GetMCSFallbackEndpoint();
+  if (fallback_endpoint.is_valid())
+    endpoints.push_back(fallback_endpoint);
   connection_factory_ = internals_builder_->BuildConnectionFactory(
       endpoints,
       GetGCMBackoffPolicy(),
       network_session_.get(),
       url_request_context_getter_->GetURLRequestContext()
           ->http_transaction_factory()
           ->GetSession(),
       &recorder_);
   connection_factory_->SetConnectionListener(this);
   mcs_client_ = internals_builder_->BuildMCSClient(
@@ skipping 879 matching lines @@
 bool GCMClientImpl::HasStandaloneRegisteredApp() const {
   if (registrations_.empty())
     return false;
   // Note that account mapper is not counted as a standalone app since it is
   // automatically started when other app uses GCM.
   return registrations_.size() > 1 ||
          !ExistsGCMRegistrationInMap(registrations_, kGCMAccountMapperAppId);
 }
 
 }  // namespace gcm