Fix cookie exclusion for fetch CORS pre-flight requests (fixes #3596)

Cookies (and other credentials) will be excluded when appropriate by
downgrading |credentials_mode| from kSameOrigin to kOmit.

Improve logic for Origin header inclusion, including a fix for
Referrer/Origin calculation in URLRequestJob::ComputeReferrerForPolicy
when used with custom standard schemes.

Specify correct CookiePartitionKeyCollection when loading cookies.

To test:
- Run tests from https://browseraudit.com/ with and without
  `--disable-request-handling-for-testing`. Results are the same.
- Run `ceftests --gtest_filter=CorsTest.*`.

libcef/common/scheme_registrar_impl.cc

@@ -49,6 +49,9 @@ bool CefSchemeRegistrarImpl::AddCustomScheme(const CefString& scheme_name,
   // with Blink only.
   if (is_standard) {
     schemes_.standard_schemes.push_back(scheme);
+    if (!is_local && !is_display_isolated) {
+      schemes_.referrer_schemes.push_back(scheme);
+    }
   }
   if (is_local) {
     schemes_.local_schemes.push_back(scheme);
@@ -74,6 +77,7 @@ bool CefSchemeRegistrarImpl::AddCustomScheme(const CefString& scheme_name,
 void CefSchemeRegistrarImpl::GetSchemes(
     content::ContentClient::Schemes* schemes) {
   AppendArray(schemes_.standard_schemes, &schemes->standard_schemes);
+  AppendArray(schemes_.referrer_schemes, &schemes->referrer_schemes);
   AppendArray(schemes_.local_schemes, &schemes->local_schemes);
   AppendArray(schemes_.secure_schemes, &schemes->secure_schemes);
   AppendArray(schemes_.cors_enabled_schemes, &schemes->cors_enabled_schemes);