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

chrome/installer/mac/app/NetworkCommunication.m

Index: chrome/installer/mac/app/NetworkCommunication.m
diff --git a/chrome/installer/mac/app/NetworkCommunication.m b/chrome/installer/mac/app/NetworkCommunication.m
index d19566a1e0188cd7a0d61bbbd0f09abc39275e02..fdee74548ecd305a22a0e0cb413d965e31de019d 100644
--- a/chrome/installer/mac/app/NetworkCommunication.m
+++ b/chrome/installer/mac/app/NetworkCommunication.m
@@ -6,6 +6,19 @@
 
 #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_;