Replace XHR events for generic ones in ScriptTask

[thiagopnts]

This refs #3735. As discussed in the issue, I did it cloning when I couldn't dereference an attribute. The trait method should be on &self for object-safety reasons.

[jdm]

Huh, does this build? What about http://mxr.mozilla.org/servo/source/components/script/dom/dedicatedworkerglobalscope.rs#134 ?

[thiagopnts]

Ops, my bad I think I forgot to run everything after removing the older XHR msgs at the end. I'll fix this and squash.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3580

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[thiagopnts]

I fixed the issue and squashed it

[jdm]

In general it's preferred to make additional commits to address review comments, and then we squash once the review is complete. However, the result here looks fine to me this time.