servo: Merge #18512 - Preserve sourceURL comment on style sheets (from tromey:preserve-style-sheet-source-url); r=SimonSapin

In addition to the sourceMappingURL comment, there is a second special
comment, "sourceURL", that can be used to set the "display name" of a
style sheet for developer tools.  This name is also used as the base
URL for the source-map URL resolution algorithm.  sourceURL is
described here:
https://blog.getfirebug.com/2009/08/11/give-your-eval-a-name-with-sourceurl/
The devtools feature bug is here:
https://bugzilla.mozilla.org/show_bug.cgi?id=880831

This patch changes servo to preserve and expose this value for use in M-C.

---
<!-- 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
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

Source-Repo: https://github.com/servo/servo
Source-Revision: 7cc0af37cfcd03b0615a408428b53d221e493570

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 02e2fccecf6516604bad0c61f5f91126c40b6f5e
extra : histedit_source : 15404491f9f20e74312b8f2f1d64a3da19ffb1b3