extended build config webchannel-wrapper to support web worker

[r0bn]

proposed fix from issue #285

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[r0bn]

I signed it!

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[r0bn]

next try (cla)

[googlebot]

CLAs look good, thanks!

[jshcrowthe]

LGTM 👍

[PCreations]

Great ! Thank you for the PR ;)

[feildmaster]

I'd like to question what uses "global"

Edit: or perhaps question the order of operations. Perhaps prioritize window would work nicer? I currently ran into an issue where I had no idea that "global" was used in any way until I discovered the closure setup. I had to go around and refactor everything just so I could use firebase.