Fix building with --feature js_backtrace
Fix building with --feature js_backtrace

- mozjs::jsapi::CapturedJSStack::as_string() (https://doc.servo.org/mozjs/rust/struct.CapturedJSStack.html#method.as_string) was updated to accept a second parameter which specifies which Stacktrace format to use. The default has been specified to preserve the (presumably) original behavior.
- The related code has also been placed in an unsafe block since the capture_stack macro calling the unsafe mozjs::jsapi::CapturedJSStack::new() function. Seems that this commit (https://github.com/servo/servo/commit/0703a1ad6d736796d467f5a028e5ab3c6d876268) forgot this part.

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