Migrate mac installer to delegate-driven model

chrome/installer/mac/app/MainDelegate.m

+// Copyright 2016 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 "MainDelegate.h"
+
+@implementation MainDelegate
+
+- (void)runApplication {
+  OmahaCommunication* messenger = [[OmahaCommunication alloc] init];
+  messenger.delegate = self;
+
+  [messenger sendRequest];
+}
+
+- (void)onOmahaSuccessWithData:(NSData*)data {
+  Downloader* download = [[Downloader alloc] init];
+  download.delegate = self;
+
+  [download downloadChromeImageToDownloadsDirectory:data];
+}
+
+- (void)onDownloadSuccess {
+  // TODO: replace the line of code below with real code someday to unpack dmg
+  exit(0);
+}
+
+@end