wasm: probe for window not for 'this'

[aep]

fixes #597

[dhardy]

Unfortunately we don't really have a maintainer for the WASM code. Please check this which does something a bit different (but doesn't appear to conflict).

Also there's a linker error due to missing libLLVM-7.so. Do you know why — do we need to update cargo-web?

[alexcrichton]

Ah unfortunately I don't think this will work 100% of the time in the sense that window isn't defined in the worker scope on the web, but something like https://github.com/rustwasm/wasm-bindgen/blob/master/crates/test/src/rt/detect.rs I think should work here.

[alexcrichton]

I believe this should be fixed in #600