SAML MGS: s/CHECK/DCHECK if command-line switch not set

CHECK should be used if the consequence of a failed assertion
would be a security vulnerability. Which does not seem to be the case.

TBR=antrim@chromium.org

Bug: 984021
Change-Id: I8c2e0137881f1c3af4b48176683b587679c0998c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865348Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706869}

chrome/browser/ui/webui/chromeos/login/gaia_screen_handler.cc

@@ -560,7 +560,7 @@ void GaiaScreenHandler::LoadGaiaWithPartitionAndVersionAndConsent(
 
   if (public_saml_url_fetcher_) {
     params.SetBoolean("startsOnSamlPage", true);
-    CHECK(base::CommandLine::ForCurrentProcess()->HasSwitch(
+    DCHECK(base::CommandLine::ForCurrentProcess()->HasSwitch(
         switches::kPublicAccountsSamlAclUrl));
     std::string saml_acl_url =
         base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(