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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Cargo.lock

@@ -1017,6 +1017,7 @@ dependencies = [
  "serde 1.0.80 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_json 1.0.13 (registry+https://github.com/rust-lang/crates.io-index)",
  "time 0.1.42 (registry+https://github.com/rust-lang/crates.io-index)",
+ "uuid 0.7.1 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]