[#26488] Refactored RTCDataChannel for safer dropping and added Promise comment (#37332)

Fixes (partially) #26488 and apply the
https://github.com/servo/servo/pull/37324#discussion_r2133989190
comment.

Testing: No tests added
Fixes: Partially #26488

---------

Signed-off-by: Domenico Rizzo <domenico.rizzo@gmail.com>

components/script/dom/promise.rs

@@ -73,6 +73,9 @@ impl PromiseHelper for Rc<Promise> {
     }
 }
 
+// Promise objects are stored inside Rc values, so Drop is run when the last Rc is dropped,
+// rather than when SpiderMonkey runs a GC. This makes it safe to interact with the JS engine unlike
+// Drop implementations for other DOM types.
 impl Drop for Promise {
     #[allow(unsafe_code)]
     fn drop(&mut self) {