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 b8bc5d3b32fdd28e5f61acb443a2610ab4e14b37..ba3e9b8bf0bcdf696e0ff48bd3cac1701a59152f 100644
--- a/ios/shared/chrome/browser/tabs/web_state_list.h
+++ b/ios/shared/chrome/browser/tabs/web_state_list.h
@@ -39,9 +39,16 @@ 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;
@@ -101,6 +108,9 @@ class WebStateList {
   // to the caller (abandon ownership of the returned WebState).
   web::WebState* DetachWebStateAt(int index) WARN_UNUSED_RESULT;
 
+  // Makes the WebState at the specified index the active WebState.
+  void ActivateWebStateAt(int index);
+
   // Adds an observer to the model.
   void AddObserver(WebStateListObserver* observer);
 
@@ -115,6 +125,10 @@ class WebStateList {
   // specified index to null.
   void ClearOpenersReferencing(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
@@ -136,6 +150,9 @@ 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);
 };