Reland "Unify allow_credentials and credentials_mode on network::ResourceRequest"

This is a reland of 1ccc5eeed0

Original change's description:
> Unify allow_credentials and credentials_mode on network::ResourceRequest
>
> Remove allow_credentials, map allow_credentials: false to
> credentials_mode: kOmit and map allow_credentials: true to
> credentials_mode: kInclude.
>
> network::URLLoader cannot handle kSameOrigin. This CL doesn't change
> that. CORSURLLoader translates the value to either kOmit or kInclude.
>
> This works correctly even when OOR-CORS is disabled because in that
> case load flags are set in the renderer. One caveat is we will not
> be able to remove the load flags until we remove the blink-side CORS
> code (M78? M79?) with this change.
>
> This CL removes a validity check for credentials related settings in
> CorsURLLoaderFactory. Originally the check was introduced to check the
> inconsistency between credentials_mode and load flags. After that
> allow_credentials was introduced, and at
> https://crrev.com/c/chromium/src/+/1443976 the logic was changed to
> check the inconsistency between credentials_mode and allow_credentials.
> Now they are merged and we don't need the check.
>
> Bug: 799935
> Change-Id: Ic05b2d41456d91fd3f48416a3a3e8fc98e235756
> Tbr: bsimonnet@chromium.org, dimich@chromium.org, groby@chromium.org, markusheintz@chromium.org, olka@chromium.org, satorux@chromium.org, tbansal@chromium.org
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1695341
> Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
> Reviewed-by: Tom Sepez <tsepez@chromium.org>
> Reviewed-by: Greg Levin <glevin@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
> Reviewed-by: Friedrich [CET] <fhorschig@chromium.org>
> Reviewed-by: Marc Treib <treib@chromium.org>
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
> Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
> Reviewed-by: Matt Menke <mmenke@chromium.org>
> Reviewed-by: Kyle Milka <kmilka@chromium.org>
> Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
> Reviewed-by: Mark Pearson <mpearson@chromium.org>
> Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
> Reviewed-by: Emily Stark <estark@chromium.org>
> Reviewed-by: John Rummell <jrummell@chromium.org>
> Reviewed-by: Ganggui Tang <gogerald@chromium.org>
> Reviewed-by: Michael Martis <martis@chromium.org>
> Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
> Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
> Reviewed-by: Patrick Noland <pnoland@chromium.org>
> Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
> Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
> Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
> Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
> Reviewed-by: David Benjamin <davidben@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#681698}

TBR=sky@chromium.org,horo@chromium.org,mpearson@chromium.org,davidben@chromium.org,thakis@chromium.org,toyoshim@chromium.org,nyquist@chromium.org,markusheintz@chromium.org,vasilii@chromium.org,jrummell@chromium.org,asvitkine@chromium.org,groby@chromium.org,bsimonnet@chromium.org,noel@chromium.org,rsorokin@chromium.org,glevin@chromium.org,yhirano@chromium.org,dimich@chromium.org,mmenke@chromium.org,nhiroki@chromium.org,sdefresne@chromium.org,tsepez@chromium.org,treib@chromium.org,estark@chromium.org,tbansal@chromium.org,gogerald@chromium.org,mattreynolds@chromium.org,wychen@chromium.org,olka@chromium.org,satorux@chromium.org,rbpotter@chromium.org,pnoland@chromium.org,fhorschig@chromium.org,martis@chromium.org,kmilka@chromium.org,jselover@chromium.org,nicolaso@chromium.org,mcarlen@chromium.org

Bug: 799935
Change-Id: Iec8067b3fed29bd6845077f5dc9c564d6640b6ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1722274
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681719}

components/safe_search_api/safe_search/safe_search_url_checker_client.cc

@@ -113,7 +113,7 @@ void SafeSearchURLCheckerClient::CheckURL(const GURL& url,
   auto resource_request = std::make_unique<network::ResourceRequest>();
   resource_request->url = GURL(kSafeSearchApiUrl);
   resource_request->method = "POST";
-  resource_request->allow_credentials = false;
+  resource_request->credentials_mode = network::mojom::CredentialsMode::kOmit;
   std::unique_ptr<network::SimpleURLLoader> simple_url_loader =
       network::SimpleURLLoader::Create(std::move(resource_request),
                                        traffic_annotation_);