Re-enable DCHECKs on amd64-generic simplechrome cros CQ bot.

The sanity test looks much more stable:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/chromeos-amd64-generic-rel-vm-tests

This should be safe to land now.

Bug: 869904
Change-Id: Ic0d7e5537e9212b7f7aaf0250041c5a73353279d
Reviewed-on: https://chromium-review.googlesource.com/1174968Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583082}

tools/mb/mb_config.pyl

@@ -554,7 +554,7 @@
     },
 
     'tryserver.chromium.chromiumos': {
-      'chromeos-amd64-generic-rel': 'cros_chrome_sdk_headless_ozone',
+      'chromeos-amd64-generic-rel': 'cros_chrome_sdk_headless_ozone_dcheck_always_on',
       'chromeos-daisy-rel': 'cros_chrome_sdk',
       'linux-chromeos-rel': 'chromeos_with_codecs_release_trybot',
       'linux-chromeos-compile-dbg': 'chromeos_with_codecs_debug_bot',