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

ios/chrome/browser/sessions/session_ios.mm

+// 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.
+
+#import "ios/chrome/browser/sessions/session_ios.h"
+
+#import "base/mac/foundation_util.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+namespace {
+// Serialization keys
+NSString* const kSessionWindowsKey = @"sessionWindows";
+}  // namespace
+
+@implementation SessionIOS
+
+@synthesize sessionWindows = _sessionWindows;
+
+#pragma mark - Public interface.
+
+- (instancetype)initWithWindows:(NSArray<SessionWindowIOS*>*)sessionWindows {
+  DCHECK(sessionWindows);
+  if ((self = [super init])) {
+    _sessionWindows = sessionWindows;
+  }
+  return self;
+}
+
+#pragma mark - NSObject
+
+- (instancetype)init {
+  return [self initWithWindows:@[]];
+}
+
+#pragma mark - NSCoding
+
+- (instancetype)initWithCoder:(NSCoder*)aDecoder {
+  NSArray<SessionWindowIOS*>* sessionWindows =
+      base::mac::ObjCCast<NSArray<SessionWindowIOS*>>(
+          [aDecoder decodeObjectForKey:kSessionWindowsKey]);
+
+  return [self initWithWindows:(sessionWindows ? sessionWindows : @[])];
+}
+
+- (void)encodeWithCoder:(NSCoder*)aCoder {
+  [aCoder encodeObject:_sessionWindows forKey:kSessionWindowsKey];
+}
+
+@end