Move the notion of current Tab from TabModel to WebStateList.

ios/shared/chrome/browser/tabs/web_state_list_observer_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/shared/chrome/browser/tabs/web_state_list_observer_bridge.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
@@ skipping 52 matching lines @@
     web::WebState* web_state,
     int index) {
   const SEL selector = @selector(webStateList:didDetachWebState:atIndex:);
   if (![observer_ respondsToSelector:selector])
     return;
 
   [observer_ webStateList:web_state_list
         didDetachWebState:web_state
                   atIndex:index];
 }
+
+void WebStateListObserverBridge::WebStateActivatedAt(
+    WebStateList* web_state_list,
+    web::WebState* old_web_state,
+    web::WebState* new_web_state,
+    int active_index,
+    bool user_action) {
+  const SEL selector = @selector
+      (webStateList:didChangeActiveWebState:oldWebState:atIndex:userAction:);
+  if (![observer_ respondsToSelector:selector])
+    return;
+
+  [observer_ webStateList:web_state_list
+      didChangeActiveWebState:new_web_state
+                  oldWebState:old_web_state
+                      atIndex:active_index
+                   userAction:(user_action ? YES : NO)];
+}