Move the notion of current Tab from TabModel to WebStateList.

ios/shared/chrome/browser/tabs/web_state_list.h

Index: ios/shared/chrome/browser/tabs/web_state_list.h
diff --git a/ios/shared/chrome/browser/tabs/web_state_list.h b/ios/shared/chrome/browser/tabs/web_state_list.h
index 6cf8472bdbbb309f29bddb874ddb1fa52133561d..6516f7afe0a383335fd25300ba2036f9aaa451f8 100644
--- a/ios/shared/chrome/browser/tabs/web_state_list.h
+++ b/ios/shared/chrome/browser/tabs/web_state_list.h
@@ -38,9 +38,16 @@ class WebStateList {
   // Returns the number of WebStates in the model.
   int count() const { return static_cast<int>(web_state_wrappers_.size()); }
 
+  // Returns the index of the currently active WebState, or kInvalidIndex if
+  // there are no active WebState.
+  int active_index() const { return active_index_; }
+
   // Returns true if the specified index is contained by the model.
   bool ContainsIndex(int index) const;
 
+  // Returns the currently active WebState or null if there is none.
+  web::WebState* GetActiveWebState() const;
+
   // Returns the WebState at the specified index. It is invalid to call this
   // with an index such that |ContainsIndex(index)| returns false.
   web::WebState* GetWebStateAt(int index) const;
@@ -99,6 +106,9 @@ class WebStateList {
   // Detaches the WebState at the specified index.
   void DetachWebStateAt(int index);
 
+  // Makes the WebState at the specified index the active WebState.
+  void ActivateWebStateAt(int index);
+
   // Adds an observer to the model.
   void AddObserver(WebStateListObserver* observer);
 
@@ -113,6 +123,10 @@ class WebStateList {
   // specified index to null.
   void FixOpenersReferencing(int index);
 
+  // Notify the observers if the active WebState change.
+  void NotifyIfActiveWebStateChanged(web::WebState* old_web_state,
+                                     bool user_action);
+
   // Returns the index of the |n|-th WebState (with n > 0) in the sequence of
   // WebStates opened from the specified WebState after |start_index|, or
   // kInvalidIndex if there are no such WebState. If |use_group| is true, the
@@ -134,6 +148,9 @@ class WebStateList {
   // List of observers notified of changes to the model.
   base::ObserverList<WebStateListObserver, true> observers_;
 
+  // Index of the currently active WebState, kInvalidIndex if no such WebState.
+  int active_index_ = kInvalidIndex;
+
   DISALLOW_COPY_AND_ASSIGN(WebStateList);
 };