Switch WebPushSender to credentials_mode.

This CL is part of a larger project to remove the LOAD_DO_NOT_* privacy
load flags and replace them with the allow_credentials setting. See
https://crbug.com/799935 and
https://docs.google.com/document/d/1ntn9N7Ce2jozvvpWI0XbzJ7lJdwUjJXK07wp7rxrIN4
for the motivation for this change.

Using credential_mode makes it harder to forget
LOAD_DO_NOT_SEND_AUTH_DATA along with the two cookie flags. This has two
implications:

- The request will no longer participate in HTTP auth if the server requests
  it. HTTP auth credentials identify the user, so private requests should
  not send them. Note this only affects server auth, not proxy auth. Unless
  the server your feature speaks to requests HTTP auth, this is a no-op.

- The request will be pooled with sockets used for uncredentialed requests,
  rather than credentialed requests. This is not expected to meaningfully
  change behavior.

If your code requires HTTP authentication, let me know. You may need to
allow credentials for your request.

Bug: 799935
Change-Id: I64c4c87a16cfb70e1ffbb5abdcc9097c48ef9956
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752745
Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Jian Li <jianli@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686598}

components/gcm_driver/web_push_sender.cc

@@ -15,7 +15,6 @@
 #include "components/gcm_driver/crypto/json_web_token_util.h"
 #include "components/gcm_driver/crypto/p256_key_util.h"
 #include "components/gcm_driver/web_push_metrics.h"
-#include "net/base/load_flags.h"
 #include "net/http/http_request_headers.h"
 #include "net/http/http_status_code.h"
 #include "services/network/public/cpp/cors/cors.h"
@@ -102,8 +101,7 @@ std::unique_ptr<network::SimpleURLLoader> BuildURLLoader(
   std::string server_url =
       base::StringPrintf(kFCMServerUrlFormat, fcm_token.c_str());
   resource_request->url = GURL(server_url);
-  resource_request->load_flags =
-      net::LOAD_DO_NOT_SEND_COOKIES | net::LOAD_DO_NOT_SAVE_COOKIES;
+  resource_request->credentials_mode = network::mojom::CredentialsMode::kOmit;
   resource_request->method = "POST";
   resource_request->headers.SetHeader(net::HttpRequestHeaders::kAuthorization,
                                       auth_header);