style: Pass a bonafide nsACString to Servo.

This will allow the Rust code hold a copy-free strong reference to the string
past callstack unwind.

Bug: 1454460
Reviewed-by: bz
MozReview-Commit-ID: HCop9h2abZU

ports/geckolib/glue.rs

@@ -1130,16 +1130,15 @@ pub extern "C" fn Servo_StyleSheet_FromUTF8Bytes(
     loader: *mut Loader,
     stylesheet: *mut ServoStyleSheet,
     load_data: *mut SheetLoadData,
-    data: *const u8,
-    data_len: usize,
+    bytes: *const nsACString,
     mode: SheetParsingMode,
     extra_data: *mut URLExtraData,
     line_number_offset: u32,
     quirks_mode: nsCompatibility,
     reusable_sheets: *mut LoaderReusableStyleSheets,
 ) -> RawServoStyleSheetContentsStrong {
     let global_style_data = &*GLOBAL_STYLE_DATA;
-    let input = unsafe { ::std::str::from_utf8_unchecked(::std::slice::from_raw_parts(data, data_len)) };
+    let input: &str = unsafe { (*bytes).as_str_unchecked() };
 
     let origin = match mode {
         SheetParsingMode::eAuthorSheetFeatures => Origin::Author,