[ios] Allow having multiple session windows in a session.

ios/chrome/browser/sessions/session_window_ios.h

 // Copyright 2012 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.
 
 #ifndef IOS_CHROME_BROWSER_SESSIONS_SESSION_WINDOW_IOS_H_
 #define IOS_CHROME_BROWSER_SESSIONS_SESSION_WINDOW_IOS_H_
 
 #import <Foundation/Foundation.h>
 
 @class CRWSessionStorage;
 
-// Encapsulates everything required to save a session "window". For iOS, there
-// will only be one window at a time.
+// Encapsulates everything required to save a session "window".
 @interface SessionWindowIOS : NSObject<NSCoding>
 
 // Initializes SessionsWindowIOS using the parameters are initial values for
 // the |sessions| and |selectedIndex| properties. |selectedIndex| must be a
 // valid indice in |sessions| or NSNotFound if |sessions| is empty.
 - (instancetype)initWithSessions:(NSArray<CRWSessionStorage*>*)sessions
                    selectedIndex:(NSUInteger)selectedIndex
     NS_DESIGNATED_INITIALIZER;
 
-// The serialized session objects.
+// The serialized session objects. May be empty but never nil.
 @property(nonatomic, readonly) NSArray<CRWSessionStorage*>* sessions;
 
 // The currently selected session. NSNotFound if the sessionWindow contains
 // no sessions; otherwise a valid index in |sessions|.
 @property(nonatomic, readonly) NSUInteger selectedIndex;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_SESSIONS_SESSION_WINDOW_IOS_H_