[ObjC ARC] Converts ios/chrome/browser/ui/toolbar:toolbar to ARC.

ios/chrome/browser/ui/toolbar/tools_menu_button_observer_bridge.mm

 // Copyright 2016 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/ui/toolbar/tools_menu_button_observer_bridge.h"
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "components/reading_list/core/reading_list_model.h"
 #import "ios/chrome/browser/ui/toolbar/toolbar_tools_menu_button.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @interface ToolsMenuButtonObserverBridge ()
 - (void)updateButtonWithModel:(const ReadingListModel*)model;
 - (void)buttonPressed:(UIButton*)sender;
 @end
 
 @implementation ToolsMenuButtonObserverBridge {
-  base::scoped_nsobject<ToolbarToolsMenuButton> _button;
+  ToolbarToolsMenuButton* _button;
   ReadingListModel* _model;
   std::unique_ptr<ReadingListModelBridge> _modelBridge;
 }
 
 - (instancetype)initWithModel:(ReadingListModel*)readingListModel
                 toolbarButton:(ToolbarToolsMenuButton*)button {
   self = [super init];
   if (self) {
-    _button.reset([button retain]);
+    _button = button;
     _model = readingListModel;
     [_button addTarget:self
                   action:@selector(buttonPressed:)
         forControlEvents:UIControlEventTouchUpInside];
     _modelBridge = base::MakeUnique<ReadingListModelBridge>(self, _model);
   }
   return self;
 }
 
 - (void)updateButtonWithModel:(const ReadingListModel*)model {
@@ skipping 25 matching lines @@
 }
 
 - (void)readingListModel:(const ReadingListModel*)model
              didAddEntry:(const GURL&)url
              entrySource:(reading_list::EntrySource)source {
   if (source == reading_list::ADDED_VIA_CURRENT_APP)
     [_button triggerAnimation];
 }
 
 @end