[mb] Fix typo in mb config

BUG=chromium:616035
TBR=kjellander@chromium.org
NOTRY=true
NOTREECHECKS=true
NOPRESUBMIT=true

Review-Url: https://codereview.chromium.org/2072833004
Cr-Commit-Position: refs/heads/master@{#37054}

infra/mb/mb_config.pyl

@@ -112,7 +112,7 @@
     },
 
     'v8_enable_slow_dchecks': {
-      'gn_args': v8_enable_slow_dchecks=true',
+      'gn_args': 'v8_enable_slow_dchecks=true',
       'gyp_defines': 'v8_enable_slow_dchecks=1',
     },