Move the notion of current Tab from TabModel to WebStateList.

ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.mm

Index: ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.mm
diff --git a/ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.mm b/ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.mm
index f2d5f720f35172a44705ebbf162422c416f3e1ad..fe5e327b551c1e993fa9df7feaae52a6f20dc84a 100644
--- a/ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.mm
+++ b/ios/shared/chrome/browser/tabs/web_state_list_observer_bridge.mm
@@ -70,3 +70,24 @@ void WebStateListObserverBridge::WebStateDetachedAt(
         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)];
+}