Add a prototype today extension behind gn flag

ios/chrome/proto_today_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/proto_today_extension/widget_view_controller.h"
+
+#import <NotificationCenter/NotificationCenter.h>
+
+#import "ios/chrome/proto_today_extension/widget_view.h"
+
+@interface WidgetViewController ()

[sdefresne, 2017/01/17 17:47:42]

Please put ARC guard:

#if !defined(__has_feature) || !__has_feature(objc_arc)
#error "This file requires ARC support."
#endif

[lody, 2017/01/18 10:48:41]

Done.

+@property(nonatomic, weak) WidgetView* widgetView;
+@end
+
+@implementation WidgetViewController
+
+@synthesize widgetView = _widgetView;
+
+#pragma mark - UIViewController
+
+- (void)viewDidLoad {
+  [super viewDidLoad];
+
+  WidgetView* widgetView = [[WidgetView alloc] init];

[sdefresne, 2017/01/17 17:47:42]

nit: you don't need the local variable

  self.widgetView = [[WidgetView alloc] init];
  [self.view addSubview:self.widgetView];

Regarding property syntax vs using ivar, I think we prefer to use the property.
Regarding property syntax vs method, I personally prefer property syntax (i.e.
"self.foo = bar;") vs method syntax (i.e. "[self setFoo:bar];").

[lody, 2017/01/18 10:48:41]

I do need it! The widgetView property is weak so if I init directly into it,
xcode tells me it's going to get immediately dealloced. Whereas here, being in a
local variable, it sticks around long enough to get retained by the addSubview
call. (Thanks stepan for helping me understand this ;) )

[sdefresne, 2017/01/18 11:11:35]

Ack. Thank you for the explanation. Can you add a comment to help future reader
understand why the local variable is necessary? Something like:

  // Use a local variable as the property is weak and the object would
  // be deallocated before it can be added as a subview.

[lody, 2017/01/18 14:56:39]

The same thing is done elsewhere (I copied this from share extension) -- I am
guessing it will be a common pattern in the arc future. But I will add the
comment here.

+  self.widgetView = widgetView;
+  [self.view addSubview:self.widgetView];
+}
+
+@end