<!-- Please describe your changes on the following line: -->
Added devtools support for fetch for XHR, but devtools can't show the request in the XHR tab. I've attached a picture of an XHR request in devtools.
<img width="694" alt="screenshot 2016-07-19 11 07 55" src="https://cloud.githubusercontent.com/assets/11877868/16944480/210b0e8a-4da1-11e6-8288-48005ede33f6.png">
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#11774 (github issue number if applicable).
<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because I don't know how to automate devtools testing for this
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
Source-Repo: https://github.com/servo/servo
Source-Revision: b5fe7db8b451f86777fb5c8d1f086319be8170c2
d9cfabcbb1
