[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];

[kkhorimoto, 2017/04/11 18:20:00]

From what I can tell, it looks like we're no longer recording state when we
switch Tabs.  It looks like WebStates record their display state when
loads/navigations occur and when the session storage is built.  If we were to
enter the Tab switcher and a WKWebView is evicted, do we have a mechanism to
restore the scroll offset when the page is reloaded?  Are we building a session
storage every time we enter the Tab Switcher?

[Eugene But (OOO till 7-30), 2017/04/11 18:39:39]

Would it be useful to commit "safe" part in one CL and then work on case by case
basis to remove |recordStateInHistory| usage from places which can potentially
break the app?

+    Tab* oldTab = LegacyTabHelper::GetTabForWebState(oldWebState);
+    DCHECK(oldTab);

[Eugene But (OOO till 7-30), 2017/04/11 14:50:48]

nit: I don't think you need to DCHECK here. |@{kTabModelTabKey : oldTab}| will
crash anyways.

 
     // 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