Add a prototype today extension behind gn flag

ios/chrome/widget_extension/widget_view_controller.mm

Index: ios/chrome/widget_extension/widget_view_controller.mm
diff --git a/ios/chrome/widget_extension/widget_view_controller.mm b/ios/chrome/widget_extension/widget_view_controller.mm
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+++ b/ios/chrome/widget_extension/widget_view_controller.mm
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+// 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];
+
+  // A local variable is necessary here as the property is declared weak and the
+  // object would be deallocated before being retained by the addSubview call.
+  WidgetView* widgetView = [[WidgetView alloc] init];
+  self.widgetView = widgetView;
+  [self.view addSubview:self.widgetView];
+}
+
+@end