Allow initializing system OTR profile.

The CHECK was introduced to trace all unexpected paths
to create OTR profile and launch incognito.
However, it didn't allow creation of system OTR profile,
e.g. for User Manager.
See crbug.com/794154 for the crash report.

BUG=794154
TEST=Run trybots

Change-Id: I4c1159fd5f25ea97fe5dacf9df3bdb4596418bb4
Reviewed-on: https://chromium-review.googlesource.com/822654Reviewed-by: Elliot Glaysher <erg@chromium.org>
Commit-Queue: Sergey Poromov <poromov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523566}

chrome/browser/profiles/off_the_record_profile_impl.cc

@@ -175,7 +175,7 @@ void OffTheRecordProfileImpl::Init() {
 
   // Always crash when incognito is not available.
   // Guest profiles may always be OTR. Check IncognitoModePrefs otherwise.
-  CHECK(profile_->IsGuestSession() ||
+  CHECK(profile_->IsGuestSession() || profile_->IsSystemProfile() ||
         IncognitoModePrefs::GetAvailability(profile_->GetPrefs()) !=
             IncognitoModePrefs::DISABLED);