Migrate mac installer to delegate-driven model

chrome/installer/mac/app/Downloader.h

 // 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.
 
 #ifndef CHROME_INSTALLER_MAC_APP_DOWNLOADER_H_
 #define CHROME_INSTALLER_MAC_APP_DOWNLOADER_H_
 
 #import <Foundation/Foundation.h>
 
-@interface Downloader : NSObject<NSXMLParserDelegate>
+@protocol DownloaderDelegate
+- (void)onDownloadSuccess;
+@end
+
+@interface Downloader
+    : NSObject<NSXMLParserDelegate, NSURLSessionDownloadDelegate>
+
+@property(nonatomic, assign) id<DownloaderDelegate> delegate;
 
 // Returns a path to a user's home download folder.
 + (NSString*)getDownloadsFilePath;
 
 - (NSMutableArray*)appendFilename:(NSString*)filename
                            toURLs:(NSArray*)incompleteURLs;
 
 // Takes an NSData with a response XML from Omaha and writes the latest
 // version of chrome to the user's download directory.
 - (BOOL)downloadChromeImageToDownloadsDirectory:(NSData*)omahaResponseXML;
 
 @end
 
 #endif  // CHROME_INSTALLER_MAC_APP_DOWNLOADER_H_