Add a prototype today extension behind gn flag

ios/chrome/proto_today_extension/widget_view.h

Index: ios/chrome/proto_today_extension/widget_view.h
diff --git a/ios/chrome/proto_today_extension/widget_view.h b/ios/chrome/proto_today_extension/widget_view.h
new file mode 100644
index 0000000000000000000000000000000000000000..748a9bfe694c85bbebf7eee4dad7c8b9aaaa7cc3
--- /dev/null
+++ b/ios/chrome/proto_today_extension/widget_view.h
@@ -0,0 +1,18 @@
+// 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.
+
+#ifndef IOS_CHROME_PROTO_TODAY_EXTENSION_WIDGET_VIEW_H_
+#define IOS_CHROME_PROTO_TODAY_EXTENSION_WIDGET_VIEW_H_
+
+#import <UIKit/UIKit.h>
+
+@interface WidgetView : UIView
+
+- (instancetype)init NS_DESIGNATED_INITIALIZER;
+- (instancetype)initWithFrame:(CGRect)frame NS_UNAVAILABLE;
+- (instancetype)initWithCoder:(NSCoder*)aDecoder NS_UNAVAILABLE;
+
+@end
+
+#endif  // IOS_CHROME_PROTO_TODAY_EXTENSION_WIDGET_VIEW_H_