Have FakeGCMProfileService return different registration IDs each time

chrome/browser/services/gcm/fake_gcm_profile_service.cc

 // Copyright 2013 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 "chrome/browser/services/gcm/fake_gcm_profile_service.h"
 
 #include "base/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "base/strings/string_number_conversions.h"
 #include "chrome/browser/profiles/profile.h"
@@ skipping 93 matching lines @@
 
 // static
 KeyedService* FakeGCMProfileService::Build(content::BrowserContext* context) {
   Profile* profile = static_cast<Profile*>(context);
   FakeGCMProfileService* service = new FakeGCMProfileService(profile);
   service->SetDriverForTesting(new CustomFakeGCMDriver(service));
   return service;
 }
 
 FakeGCMProfileService::FakeGCMProfileService(Profile* profile)
-    : collect_(false) {
+    : collect_(false),
+      registration_count_(0) {
   static_cast<PushMessagingServiceImpl*>(push_messaging_service())
       ->SetProfileForTesting(profile);
 }
 
 FakeGCMProfileService::~FakeGCMProfileService() {}
 
 void FakeGCMProfileService::RegisterFinished(
     const std::string& app_id,
     const std::vector<std::string>& sender_ids) {
   if (collect_) {
     last_registered_app_id_ = app_id;
     last_registered_sender_ids_ = sender_ids;
   }
 
   CustomFakeGCMDriver* custom_driver =
       static_cast<CustomFakeGCMDriver*>(driver());
   custom_driver->OnRegisterFinished(app_id,
-                           base::UintToString(sender_ids.size()),
+                           base::UintToString(1000 * (sender_ids.size() - 1)
+                                              + (++registration_count_)),

[Avi (use Gerrit), 2014/12/08 19:08:43]

Put your explanation for this directly into the code as a comment.

[johnme, 2014/12/08 20:30:28]

Done.

                            GCMClient::SUCCESS);
 }
 
 void FakeGCMProfileService::UnregisterFinished(const std::string& app_id) {
   GCMClient::Result result = GCMClient::SUCCESS;
   if (!unregister_responses_.empty()) {
     result = unregister_responses_.front();
     unregister_responses_.pop_front();
   }
 
@@ skipping 23 matching lines @@
     GCMClient::Result result) {
   unregister_responses_.push_back(result);
 }
 
 void FakeGCMProfileService::SetUnregisterCallback(
     const UnregisterCallback& callback) {
   unregister_callback_ = callback;
 }
 
 }  // namespace gcm