[Extensions] Don't allow gin::Define to be overridden

Use DefineOwnProperty instead of Set in for gin, including gin::Define.
Replace Set in v8_helpers as well, to avoid the same problem.
Also update callsites from JS to CHECK expected arguments, rather
than DCHECK (since receiving unexpected arguments likely means
executing untrusted code).


Review URL: https://codereview.chromium.org/1433293004

Cr-Commit-Position: refs/heads/master@{#359460}
8 files changed