Fix problem in Firefox webextension content script

[frankier]

No description provided.

[dianlight]

Hi,
I try your branch and in firefox webext works great. But if you try in a pure node environment fails because the "window" is not defined.

I suggest you to test first typeof window and next it it differ from this.

Regards
L.

[conoremclaughlin]

For anyone that finds their way here from crashes in a Firefox extension environment, I've merged this branch with 1.3.0 here:
conoremclaughlin@9a5223f

You can fork it and use it for solving any reflect-metadata problems you may have in Firefox. Good luck!

[rbuckton]

I'm not comfortable taking this PR as I'm not familiar with what window.wrappedJSObject is, and how this change might affect other consumers of the library. The test seems fairly fragile as well, as indicated by @dianlight's comment.

I'd like to understand more about what the specific issue is to determine whether this change is acceptable, or if there is another change that could be made in its place. If this is still a concern, please file or provide a link to an issue regarding the underlying cause for further discussion.