Bug 1762521 - Add further locking to JumpListBuilder. r=mhowell,jesup

This fixes two cases where thread safety analysis complains about us not holding
the monitor.

Differential Revision: https://phabricator.services.mozilla.com/D142775

widget/windows/JumpListBuilder.cpp

@@ -107,6 +107,7 @@ JumpListBuilder::JumpListBuilder()
       return;
     }
 
+    ReentrantMonitorAutoEnter lock(mMonitor);
     // Since we are accessing mJumpListMgr across different threads
     // (ie, different apartments), mJumpListMgr must be an agile reference.
     mJumpListMgr = jumpListMgr;
@@ -140,6 +141,7 @@ JumpListBuilder::~JumpListBuilder() {
 
 NS_IMETHODIMP JumpListBuilder::SetAppUserModelID(
     const nsAString& aAppUserModelId) {
+  ReentrantMonitorAutoEnter lock(mMonitor);
   if (!mJumpListMgr) return NS_ERROR_NOT_AVAILABLE;
 
   RefPtr<ICustomDestinationList> jumpListMgr = mJumpListMgr;