Auto merge of #12502 - avadacatavra:devtools, r=jdm

Added devtools support to fetch for XHR

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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">

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<!-- 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).

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- [ ] 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

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