Let leakWarning carry the string representation, not the stack itself (…

…#22488)

At least with recent Chrome, I ran into a situation where an object of
mine apparently didn't get garbage collected because it was retained via
such a leakWarning stack trace. (I'm not sure about the internal
workings of that, though; but the object started to get garbage
collected once I did this change.)

I have no idea if there are any other up-/downsides to carrying the
stack vs. just the string here. And my explanation of what exactly it
fixed for me is probably a bit vague. But wanted to bring it up
nevertheless---maybe it is considered a good change anyway.

src/embind/embind.js

@@ -1522,7 +1522,7 @@ var LibraryEmbind = {
     // at run-time, not build-time.
     finalizationRegistry = new FinalizationRegistry((info) => {
 #if ASSERTIONS
-      console.warn(info.leakWarning.stack.replace(/^Error: /, ''));
+      console.warn(info.leakWarning);
 #endif
       releaseClassHandle(info.$$);
     });
@@ -1538,13 +1538,14 @@ var LibraryEmbind = {
         // This is more useful than the empty stacktrace of `FinalizationRegistry`
         // callback.
         var cls = $$.ptrType.registeredClass;
-        info.leakWarning = new Error(`Embind found a leaked C++ instance ${cls.name} <${ptrToString($$.ptr)}>.\n` +
+        var err = new Error(`Embind found a leaked C++ instance ${cls.name} <${ptrToString($$.ptr)}>.\n` +
         "We'll free it automatically in this case, but this functionality is not reliable across various environments.\n" +
         "Make sure to invoke .delete() manually once you're done with the instance instead.\n" +
         "Originally allocated"); // `.stack` will add "at ..." after this sentence
         if ('captureStackTrace' in Error) {
-          Error.captureStackTrace(info.leakWarning, RegisteredPointer_fromWireType);
+          Error.captureStackTrace(err, RegisteredPointer_fromWireType);
         }
+        info.leakWarning = err.stack.replace(/^Error: /, '');
 #endif
         finalizationRegistry.register(handle, info, handle);
       }