Bug 1749051: compositor cleanup r=jnicol

Differential Revision: https://phabricator.services.mozilla.com/D135406

gfx/layers/ipc/CompositorBridgeParent.cpp

@@ -241,6 +241,7 @@ void CompositorBridgeParent::FinishShutdown() {
   }
 
   // TODO: this should be empty by now...
+  MonitorAutoLock lock(*sIndirectLayerTreesLock);
   sIndirectLayerTrees.clear();
 }