Add a prototype today extension behind gn flag

ios/chrome/proto_today_extension/widget_view.mm

Index: ios/chrome/proto_today_extension/widget_view.mm
diff --git a/ios/chrome/proto_today_extension/widget_view.mm b/ios/chrome/proto_today_extension/widget_view.mm
new file mode 100644
index 0000000000000000000000000000000000000000..06928f2792f613b7c1f3c543d163f25bb0d87241
--- /dev/null
+++ b/ios/chrome/proto_today_extension/widget_view.mm
@@ -0,0 +1,16 @@
+// 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/proto_today_extension/widget_view.h"
+
+@implementation WidgetView
+
+#pragma mark - Lifecycle
+
+- (instancetype)init {
+  self = [super initWithFrame:CGRectZero];
+  return self;
+}
+
+@end