Enable the AllowSuperUnsafeScript assert disabler in debug builds.

Don't crash debug builds when there's an AllowSuperUnsafeScript assert
disabler in place.

R=haraken@chromium.org,abarth@chromium.org
BUG=395164

Review URL: https://codereview.chromium.org/402303003

git-svn-id: svn://svn.chromium.org/blink/trunk@178615 bbb929c8-8fbe-4397-9dbb-9b2b20218538