[ios] Style cleanups in tab switcher code.

ios/chrome/browser/ui/tab_switcher/tab_switcher_model.mm

 // Copyright 2015 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.
 
 #import "ios/chrome/browser/ui/tab_switcher/tab_switcher_model.h"
 
 #include <memory>
 
 #include "base/memory/ptr_util.h"
 #include "components/browser_sync/profile_sync_service.h"
@@ skipping 131 matching lines @@
 
 - (NSInteger)distantSessionCount {
   return _syncedSessions->GetSessionCount();
 }
 
 - (ios::ChromeBrowserState*)browserState {
   return _browserState;
 }
 
 - (TabModel*)tabModelForSessionOfType:(TabSwitcherSessionType)type {
-  DCHECK(type == TabSwitcherSessionType::OFF_THE_RECORD_SESSION ||
-         type == TabSwitcherSessionType::REGULAR_SESSION);
+  DCHECK(TabSwitcherSessionTypeIsLocalSession(type));
   return type == TabSwitcherSessionType::OFF_THE_RECORD_SESSION ? _otrTabModel
                                                                 : _mainTabModel;
 }
 
 - (NSInteger)numberOfTabsInLocalSessionOfType:(TabSwitcherSessionType)type {
   TabModelSnapshot* tabModelSnapshot = [self tabModelSnapshotForSession:type];
   return tabModelSnapshot->tabs().size();
 }
 
 - (Tab*)tabAtIndex:(NSUInteger)index
@@ skipping 230 matching lines @@
   [self tabModelChanged:model];
 }
 
 - (void)tabModel:(TabModel*)model
     didChangeTabSnapshot:(Tab*)tab
                withImage:(UIImage*)image {
   [self tabModelChanged:model];
 }
 
 @end