Fix DownloadTaskImplTest for iOS 12.

iOS 12 calls private undocumented method from NSHTTPCookieStorage, which
needs to be stubbed in CRWFakeNSURLSessionTask in order to use fake task
with NSHTTPCookieStorage.

Bug: 851472
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I59b676090786a65e902ccd98a5fea85e2b191fed
Reviewed-on: https://chromium-review.googlesource.com/1097570Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566577}

ios/web/test/fakes/crw_fake_nsurl_session_task.mm

@@ -39,10 +39,14 @@
   self.state = NSURLSessionTaskStateRunning;
 }
 
-// A private method, called by -[NSHTTPCookieStorage storeCookies:forTask:].
-// Requires stubbing in order to use NSHTTPCookieStorage API.
+// Below are private methods, called by
+// -[NSHTTPCookieStorage storeCookies:forTask:]. Require stubbing in order to
+// use NSHTTPCookieStorage API.
 - (NSString*)_storagePartitionIdentifier {
   return nil;
 }
+- (NSURL*)_siteForCookies {
+  return nil;
+}
 
 @end