Add a prototype today extension behind gn flag

ios/chrome/widget_extension/widget_view_controller.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/widget_extension/widget_view_controller.h"
+
+#import <NotificationCenter/NotificationCenter.h>
+
+#import "ios/chrome/widget_extension/widget_view.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+@interface WidgetViewController ()
+@property(nonatomic, weak) WidgetView* widgetView;
+@end
+
+@implementation WidgetViewController
+
+@synthesize widgetView = _widgetView;
+
+#pragma mark - UIViewController
+
+- (void)viewDidLoad {
+  [super viewDidLoad];
+
+  WidgetView* widgetView = [[WidgetView alloc] init];
+  self.widgetView = widgetView;
+  [self.view addSubview:self.widgetView];
+}
+
+@end