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

ios/chrome/browser/sessions/session_ios.h

+// 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.
+
+#ifndef IOS_CHROME_BROWSER_SESSIONS_SESSION_IOS_H_
+#define IOS_CHROME_BROWSER_SESSIONS_SESSION_IOS_H_
+
+#import <Foundation/Foundation.h>
+
+@class SessionWindowIOS;
+
+// Encapsulates everything required to save a session. A session is a set of
+// one or more session windows that share the same browser state.
+@interface SessionIOS : NSObject<NSCoding>
+
+- (instancetype)initWithWindows:(NSArray<SessionWindowIOS*>*)sessionWindows
+    NS_DESIGNATED_INITIALIZER;
+
+// The serialized SessionWindowIOS objects. May be empty but never nil.
+@property(nonatomic, readonly) NSArray<SessionWindowIOS*>* sessionWindows;
+
+@end
+
+#endif  // IOS_CHROME_BROWSER_SESSIONS_SESSION_IOS_H_