[iOS Clean] Added dialogs UI support.

ios/clean/chrome/browser/ui/dialogs/dialog_button_item.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/clean/chrome/browser/ui/dialogs/dialog_button_item.h"
+
+#include "base/logging.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+@interface DialogButtonItem ()
+
+// Initializer used by the factory method.
+- (instancetype)initWithText:(NSString*)text
+                       style:(DialogButtonStyle)style
+                         tag:(id)tag NS_DESIGNATED_INITIALIZER;
+
+@end
+
+@implementation DialogButtonItem
+
+@synthesize text = _text;
+@synthesize style = _style;
+@synthesize tag = _tag;
+
+- (instancetype)initWithText:(NSString*)text
+                       style:(DialogButtonStyle)style
+                         tag:(id)tag {
+  DCHECK(text.length);
+  DCHECK(tag);
+  if ((self = [super init])) {
+    _text = text;

[marq (ping after 24h), 2017/06/14 10:52:15]

[text copy]

[kkhorimoto, 2017/06/23 06:24:18]

Done.

+    _style = style;
+    _tag = tag;

[marq (ping after 24h), 2017/06/14 10:52:15]

using a totally opaque type is slightly tricky here -- what if it's mutable,
retained by another object, and changes? Really we want this to be some kind of
opaque *value* type. 

[kkhorimoto, 2017/06/23 06:24:17]

The identifier is supplied by and sent back to the coordinator layer.  Since
it's supplied to the consumer with no type information, only the coordinator can
actually modify the identifier object, so I think it's reasonable to expect that
if a coordinator modifies the identifier somehow, it does it in a way in which
the buttons can still be uniquely identified.  Moreover, even if the objects
supplied are mutable, it wouldn't really make a different in the current
implementation, since it's relying on direct pointer comparison rather than any
selectors.

+  }
+  return self;
+}
+
+#pragma mark - Public
+
++ (instancetype)itemWithText:(NSString*)text
+                       style:(DialogButtonStyle)style
+                         tag:(id)tag {
+  return [[[self class] alloc] initWithText:text style:style tag:tag];
+}
+
+@end