Auto merge of #23296 - codehag:evaluate-js-fix, r=Manishearth

Enable evaluating javascript in the firefox devtools console

This enables one to use the Firefox Devtools console to evaluate things in servo.

Steps:
start servo: ./mach run tests/html/about-mozilla.html -d --devtools 6000
start firefox and go to Tools -> Web Developer -> Connect...
Set the port to connect to as 6000
Try evaluating things in the console.

<img width="901" alt="Screenshot 2019-05-01 at 13 02 37" src="https://user-images.githubusercontent.com/26968615/57015054-0aa0ce80-6c13-11e9-8031-9299f2da5dba.png">

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