PerUserTopicSubscriptionRequest: set credentials_mode to kOmit

The request doesn't actually need cookies and already has
cookies_allowed policy set to NO. To match this credentials_mode now
set to kOmit.

Bug: 1020117
Change-Id: I7c9198bc388f890492c07334492e96f14f3776a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2049858
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740640}

components/invalidation/impl/per_user_topic_subscription_request.cc

@@ -14,6 +14,7 @@
 #include "components/sync/base/model_type.h"
 #include "net/http/http_status_code.h"
 #include "net/url_request/url_fetcher.h"
+#include "services/network/public/mojom/fetch_api.mojom-shared.h"
 
 using net::HttpRequestHeaders;
 
@@ -369,8 +370,8 @@ PerUserTopicSubscriptionRequest::Builder::BuildURLFetcher(
   }
   request->url = url;
   request->headers = headers;
-  // TODO(crbug.com/1020117): Should we set request->credentials_mode to kOmit,
-  // to match "cookies_allowed: NO" above?
+  // Disable cookies for this request.
+  request->credentials_mode = network::mojom::CredentialsMode::kOmit;
 
   std::unique_ptr<network::SimpleURLLoader> url_loader =
       network::SimpleURLLoader::Create(std::move(request), traffic_annotation);