Redeclare symbols to fix partial availability warnings in //chrome/installer/mac/.

chrome/installer/mac/app/NetworkCommunication.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 <Foundation/Foundation.h>
 
 #import "NetworkCommunication.h"
 
+@interface NSURLSession (PartialAvailability)
+- (NSURLSessionDataTask*)dataTaskWithRequest:(NSURLRequest*)request
+                           completionHandler:
+                               (void (^)(NSData* data,
+                                         NSURLResponse* response,
+                                         NSError* error))completionHandler;
+- (NSURLSessionDownloadTask*)
+downloadTaskWithRequest:(NSURLRequest*)request
+      completionHandler:(void (^)(NSURL* location,
+                                  NSURLResponse* response,
+                                  NSError* error))completionHandler;
+@end
+
 @implementation NetworkCommunication : NSObject
 
 @synthesize session = session_;
 @synthesize request = request_;
 @synthesize dataResponseHandler = dataResponseHandler_;
 @synthesize downloadResponseHandler = downloadResponseHandler_;
 
 - (id)init {
   return [self initWithDelegate:nil];
 }
@@ skipping 37 matching lines @@
   if (downloadResponseHandler_) {
     downloadTask = [session_ downloadTaskWithRequest:request_
                                    completionHandler:downloadResponseHandler_];
   } else {
     downloadTask = [session_ downloadTaskWithRequest:request_];
   }
   [downloadTask resume];
 }
 
 @end