TpmChallenge: check EnabledForUser policy level

Enable the check that AttestationEnabledForUser policy
is configured from cloud and the level is "Mandatory".

This CL reverts https://crrev.com/c/1865340 because
the values in the corp policies were fixed.

Bug: 1000589
Test: *TpmChallenge*
Change-Id: I17058e673100673023c3e8f0f759e0b4b92afac6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436822
Commit-Queue: Michael Ershov <miersh@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811663}

chrome/browser/chromeos/attestation/tpm_challenge_key_subtle.cc

@@ -248,8 +248,7 @@ bool TpmChallengeKeySubtleImpl::IsRemoteAttestationEnabledForUser() const {
   DCHECK(profile_);
 
   PrefService* prefs = profile_->GetPrefs();
-  // TODO(crbug.com/1000589): Check it's mandatory after fixing corp policy.
-  if (prefs) {
+  if (prefs && prefs->IsManagedPreference(prefs::kAttestationEnabled)) {
     return prefs->GetBoolean(prefs::kAttestationEnabled);
   }
   return false;