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

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

Index: ios/chrome/browser/ui/toolbar/tools_menu_button_observer_bridge.mm
diff --git a/ios/chrome/browser/ui/toolbar/tools_menu_button_observer_bridge.mm b/ios/chrome/browser/ui/toolbar/tools_menu_button_observer_bridge.mm
index 9f61118051d1daf8f179a6b94a3e536614546378..b7e76e78779f3c68fc5f44988c242d144ab05e09 100644
--- a/ios/chrome/browser/ui/toolbar/tools_menu_button_observer_bridge.mm
+++ b/ios/chrome/browser/ui/toolbar/tools_menu_button_observer_bridge.mm
@@ -9,13 +9,17 @@
 #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;
 }
@@ -24,7 +28,7 @@
                 toolbarButton:(ToolbarToolsMenuButton*)button {
   self = [super init];
   if (self) {
-    _button.reset([button retain]);
+    _button = button;
     _model = readingListModel;
     [_button addTarget:self
                   action:@selector(buttonPressed:)