Move the notion of current Tab from TabModel to WebStateList.

ios/chrome/browser/tabs/tab_model_observers_bridge.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_observers_bridge.h"
 
 #include "base/logging.h"
 #import "ios/chrome/browser/tabs/legacy_tab_helper.h"
 #import "ios/chrome/browser/tabs/tab_model.h"
 #import "ios/chrome/browser/tabs/tab_model_observers.h"
@@ skipping 57 matching lines @@
 - (void)webStateList:(WebStateList*)webStateList
     didDetachWebState:(web::WebState*)webState
               atIndex:(int)index {
   DCHECK_GE(index, 0);
   [_tabModelObservers tabModel:_tabModel
                   didRemoveTab:LegacyTabHelper::GetTabForWebState(webState)
                        atIndex:static_cast<NSUInteger>(index)];
   [_tabModelObservers tabModelDidChangeTabCount:_tabModel];
 }
 
+- (void)webStateList:(WebStateList*)webStateList
+    didChangeActiveWebState:(web::WebState*)newWebState
+                oldWebState:(web::WebState*)oldWebState
+                    atIndex:(int)atIndex
+                 userAction:(BOOL)userAction {
+  DCHECK_GE(atIndex, 0);
+  if (!newWebState)
+    return;
+
+  Tab* oldTab =
+      oldWebState ? LegacyTabHelper::GetTabForWebState(oldWebState) : nil;
+  [_tabModelObservers tabModel:_tabModel
+            didChangeActiveTab:LegacyTabHelper::GetTabForWebState(newWebState)
+                   previousTab:oldTab
+                       atIndex:static_cast<NSUInteger>(atIndex)];
+}
+
 @end