Bug 1717728 - Hold MessageChannel's monitor when creating PortLink, r=handyman

Differential Revision: https://phabricator.services.mozilla.com/D118562

ipc/glue/MessageLink.cpp

@@ -73,6 +73,8 @@ class PortLink::PortObserverThunk : public NodeController::PortObserver {
 
 PortLink::PortLink(MessageChannel* aChan, ScopedPort aPort)
     : MessageLink(aChan), mNode(aPort.Controller()), mPort(aPort.Release()) {
+  MonitorAutoLock lock(*mChan->mMonitor);
+
   mObserver = new PortObserverThunk(mChan->mMonitor, this);
   mNode->SetPortObserver(mPort, mObserver);