Create a FakeTetherService, which stubs out TetherService for development.

chrome/browser/chromeos/login/session/user_session_manager.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 "chrome/browser/chromeos/login/session/user_session_manager.h"
 
 #include <stddef.h>
 
 #include <set>
 #include <string>
@@ skipping 1207 matching lines @@
   const user_manager::User* user =
       ProfileHelper::Get()->GetUserByProfile(profile);
   if (user_manager->GetPrimaryUser() == user) {
     InitRlz(profile);
     InitializeCerts(profile);
     InitializeCRLSetFetcher(user);
     InitializeCertificateTransparencyComponents(user);
 
     arc::ArcServiceLauncher::Get()->OnPrimaryUserProfilePrepared(profile);
 
-    TetherService::Get(profile)->StartTetherIfEnabled();
+    TetherService* tether_service = TetherService::Get(profile);
+    if (tether_service)
+      tether_service->StartTetherIfEnabled();
   }
 
   UpdateEasyUnlockKeys(user_context_);
   user_context_.ClearSecrets();
   if (TokenHandlesEnabled()) {
     CreateTokenUtilIfMissing();
     if (token_handle_util_->ShouldObtainHandle(user->GetAccountId())) {
       if (!token_handle_fetcher_.get()) {
         token_handle_fetcher_.reset(new TokenHandleFetcher(
             token_handle_util_.get(), user->GetAccountId()));
@@ skipping 700 matching lines @@
           ->browser_policy_connector_chromeos()
           ->IsEnterpriseManaged()) {
     return false;
   }
 
   // Do not show end of life notification if this is a guest session
   return !profile->IsGuestSession();
 }
 
 }  // namespace chromeos