Local NTP: just send credentials with promo service requests

This was previously the case, at least for server-side opt-in to work.
Somewhere along the line this was changed (multiple times) and now
server-side opt-in to show "middle-slot" promos doesn't work any more.
This is a pain for developers but a bigger pain for less technical folks
to show demo promos.

Also, OGB and doodle requests already send creds as well.

R=mahmadi@chromium.org
BUG=1003508

Change-Id: I75869a818f7fcaeba08fa3babbfa30426fb06cce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830365Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700936}

chrome/browser/search/promos/promo_service.cc

@@ -8,6 +8,7 @@
 
 #include "base/bind.h"
 #include "base/callback.h"
+#include "base/feature_list.h"
 #include "base/values.h"
 #include "chrome/browser/search/ntp_features.h"
 #include "chrome/common/pref_names.h"
@@ -144,7 +145,6 @@ void PromoService::Refresh() {
 
   auto resource_request = std::make_unique<network::ResourceRequest>();
   resource_request->url = GetApiUrl();
-  resource_request->credentials_mode = network::mojom::CredentialsMode::kOmit;
   resource_request->request_initiator =
       url::Origin::Create(GURL(chrome::kChromeUINewTabURL));