Set 60 seconds as timeoutIntervalForResource for Downloads.

Background NSURLSession never fails due to connectivity errors. Instead
that session waits for connectivity up to timeoutIntervalForResource
timeout which defaults to 1 week.

This CL changes the timeout to 60 seconds.

Bug: 824018
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I841fc4580fa6bc6724198d825bb1736b91027fc4
Reviewed-on: https://chromium-review.googlesource.com/976623Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545482}

ios/web/download/download_controller_impl.mm

@@ -88,6 +88,9 @@ NSURLSession* DownloadControllerImpl::CreateSession(
     NSOperationQueue* delegate_queue) {
   NSURLSessionConfiguration* configuration = [NSURLSessionConfiguration
       backgroundSessionConfigurationWithIdentifier:identifier];
+  // Background URL Session will not fail and will wait for connectivity up
+  // until this timeout (which defaults to one week).
+  configuration.timeoutIntervalForResource = 60;
 
   std::string user_agent = GetWebClient()->GetUserAgent(UserAgentType::MOBILE);
   configuration.HTTPAdditionalHeaders = @{