Expose the source map URL on a style sheet

This changes Servo to track the source map URL of a style sheet.  This
parallels a change going in to Gecko:
https://bugzilla.mozilla.org/show_bug.cgi?id=1388855

components/style/gecko/generated/bindings.rs

@@ -1960,6 +1960,10 @@ extern "C" {
                                           RawServoStyleSheetContentsBorrowed)
      -> OriginFlags;
 }
+extern "C" {
+    pub fn Servo_StyleSheet_GetSourceMapURL(sheet: RawServoStyleSheetContentsBorrowed,
+                                            result: *mut nsAString);
+}
 extern "C" {
     pub fn Servo_StyleSet_Init(pres_context: RawGeckoPresContextOwned)
      -> *mut RawServoStyleSet;