Auto merge of #21945 - codehag:devtools-add-emulation-actor, r=jdm

DevTools - add Emulation Actor

Last one for today, this one adds the Emulation actor, which allows the netmonitor panel to load. The emulation actor has no methods for now, but will eventually be used by the netmonitor to throttle connections.

Currently the netmonitor shows but doesn't have any values. will get to that soon!

<img width="901" alt="screen shot 2018-10-14 at 19 26 16" src="https://user-images.githubusercontent.com/26968615/46920018-30a0b180-cfe8-11e8-83ec-51ca71f0e8c3.png">

Combined with the changes for the stylesheets actor, the device actor, and the extra methods added in browserContext and threadActor, the devtools are now all loading \o/

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