Auto merge of #18512 - tromey:preserve-style-sheet-source-url, r=SimonSapin

Preserve sourceURL comment on style sheets

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.

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

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components/style/gecko/generated/bindings.rs

@@ -1977,6 +1977,10 @@ extern "C" {
                                                 RawServoStyleSheetContentsBorrowed,
                                             result: *mut nsAString);
 }
+extern "C" {
+    pub fn Servo_StyleSheet_GetSourceURL(sheet: RawServoStyleSheetContentsBorrowed,
+                                         result: *mut nsAString);
+}
 extern "C" {
     pub fn Servo_StyleSheet_GetOrigin(sheet:
                                           RawServoStyleSheetContentsBorrowed)