handle undefined document, window variables in BrowserPlatform

[Bunkerbewohner]

Simple fix for the problem described in #1138

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[Bunkerbewohner]

I signed the CLA!

[googlebot]

CLAs look good, thanks!

[wilhuff]

In principle this seems like a good change, thanks for this!

@schmidt-sebastian Could you double-check that this is right?

[schmidt-sebastian]

Thanks for sending this over. I would like to go one step further and verify that "window" and "document" are known types before accessing them. I have seen some platforms (such as Node) that error even when checks like "window || null" are performed.

See follow-up PR #1140.

[Bunkerbewohner]

Ooops seems like I accidently pushed the wrong code and didn't even notice it. My bad. You are of course right that just ORing with null is not sufficient. Thanks for the follow up PR!

I updated my PR just for the sake of correctness.

[schmidt-sebastian]

Thanks for updating this. I have merged my follow-up PR, which uses the same logic you have added in this PR. I will close this PR as #1140 is merged. Thanks!