Bug 1759572 - AsyncLengthWaitHelper should be threadsafe, r=asuth

This works only because the only implementation of nsIAsyncInputStreamLength
happened to only run on the current thread. Bug 1754004, changes this, and requires the reference to be threadsafe.

Differential Revision: https://phabricator.services.mozilla.com/D141037

xpcom/io/nsMultiplexInputStream.cpp

@@ -1276,11 +1276,9 @@ nsMultiplexInputStream::Length(int64_t* aLength) {
 }
 
 class nsMultiplexInputStream::AsyncWaitLengthHelper final
-    : public nsIInputStreamLengthCallback
-
-{
+    : public nsIInputStreamLengthCallback {
  public:
-  NS_DECL_ISUPPORTS
+  NS_DECL_THREADSAFE_ISUPPORTS
 
   AsyncWaitLengthHelper()
       : mStreamNotified(false), mLength(0), mNegativeSize(false) {}