Bug 1937694, don't try to keep transferred ports alive for too long, r=mccr8,sfink

Differential Revision: https://phabricator.services.mozilla.com/D232405

dom/base/StructuredCloneHolder.cpp

@@ -390,6 +390,7 @@ void StructuredCloneHolder::Read(nsIGlobalObject* aGlobal, JSContext* aCx,
   mGlobal = aGlobal;
 
   if (!StructuredCloneHolderBase::Read(aCx, aValue, aCloneDataPolicy)) {
+    mTransferredPorts.Clear();
     JS_ClearPendingException(aCx);
     aRv.ThrowDataCloneError(mErrorMessage);
     return;