[ios] Change WebStateList serialization method to use SessionWindowIOS.

ios/chrome/browser/sessions/NSCoder+Compatibility.mm

 // Copyright 2014 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/NSCoder+Compatibility.h"
 
 #include "base/logging.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 // Note: |NSNotFound| is equal to |NSIntegerMax| in 32-bit and 64-bit that
 // in turn is initialized to |LONG_MAX|. On a 32-bit build, |INT_MAX| and
 // |LONG_MAX| have the same value, however, in a 64-bit build, |LONG_MAX|
 // is much larger, so we define |NSNotFound32| to |INT_MAX| that has the
 // same value in both 32-bit and 64-bit builds.
 enum { NSNotFound32 = INT_MAX };
 }  // namespace
 
 @implementation NSCoder (Compatibility)
 
 - (NSInteger)cr_decodeIndexForKey:(NSString*)key {
   int32_t index32 = [self decodeInt32ForKey:key];
   DCHECK(0 <= index32 && index32 <= NSNotFound32);
   return index32 == NSNotFound32 ? NSNotFound : static_cast<NSInteger>(index32);
 }
 
 - (void)cr_encodeIndex:(NSInteger)index forKey:(NSString*)key {
   DCHECK((0 <= index && index < NSNotFound32) || index == NSNotFound);
   int32_t index32 = index == NSNotFound ? static_cast<int32_t>(NSNotFound32)
                                         : static_cast<int32_t>(index);
   [self encodeInt32:index32 forKey:key];
 }
 
 @end