Auto merge of #21944 - codehag:devtools-add-thread-interrupt, r=jdm

DevTools - add Interrupt method to ThreadActor

This is one of three pull requests that allows the DevTools Debugger to render.

The two related prs are #21942 and #21943

In this pr, I introduced the `interrupt` method to the threadActor. This is arguably a carry-over from the firefox devtools, where the debugger server is running in the same event loop as the content scripts, and it can only update itself when the debugger server is paused. Depending on how debugging is, or how debugging will be implemented on servo, this method may need to be adjusted for "debugger events" and "script events".

After all three patches on this topic are merged, you should be able to see the debugger \o/ (but no sources yet)

<img width="900" alt="screen shot 2018-10-14 at 16 57 07" src="https://user-images.githubusercontent.com/26968615/46918408-35f30180-cfd2-11e8-9a98-8e1540adc894.png">

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