[ios] Reduce access to private //ios/web API.

ios/chrome/browser/tabs/tab_model_selected_tab_observer.mm

 // Copyright 2017 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/tabs/tab_model_selected_tab_observer.h"
 
 #include "base/logging.h"
 #import "ios/chrome/browser/tabs/legacy_tab_helper.h"
 #import "ios/chrome/browser/tabs/tab.h"
 #import "ios/chrome/browser/tabs/tab_model.h"
@@ skipping 14 matching lines @@
   return self;
 }
 
 #pragma mark WebStateListObserving
 
 - (void)webStateList:(WebStateList*)webStateList
     didChangeActiveWebState:(web::WebState*)newWebState
                 oldWebState:(web::WebState*)oldWebState
                     atIndex:(int)atIndex
                  userAction:(BOOL)userAction {
-  Tab* oldTab = nil;
-  Tab* newTab = nil;
   if (oldWebState) {
     // Save state, such as scroll position, ... of the old selected Tab.
-    oldTab = LegacyTabHelper::GetTabForWebState(oldWebState);
-    if (userAction)
-      [oldTab recordStateInHistory];
+    Tab* oldTab = LegacyTabHelper::GetTabForWebState(oldWebState);
+    DCHECK(oldTab);
 
     // Avoid artificially extending the lifetime of oldTab until the global
     // autoreleasepool is purged.
     @autoreleasepool {
       [[NSNotificationCenter defaultCenter]
           postNotificationName:kTabModelTabDeselectedNotification
                         object:_tabModel
                       userInfo:@{kTabModelTabKey : oldTab}];
     }
   }
 
   if (newWebState) {
-    newTab = LegacyTabHelper::GetTabForWebState(newWebState);
+    Tab* newTab = LegacyTabHelper::GetTabForWebState(newWebState);
     [newTab updateLastVisitedTimestamp];
 
     // Persist the session state.
     if (newTab.loadFinished)
       [_tabModel saveSessionImmediately:NO];
   }
 }
 
 @end