Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/engine/passive_model_worker.cc

Index: components/sync/engine/passive_model_worker.cc
diff --git a/components/sync/engine/passive_model_worker.cc b/components/sync/engine/passive_model_worker.cc
index 2ea6edf9a69ff3a4b371999c5b75e00442d965d9..623b31961526b5d5ec0bc078dc1d64c324bdbfd2 100644
--- a/components/sync/engine/passive_model_worker.cc
+++ b/components/sync/engine/passive_model_worker.cc
@@ -4,19 +4,14 @@
 
 #include "components/sync/engine/passive_model_worker.h"
 
-#include "base/message_loop/message_loop.h"
+#include "base/callback.h"
 
 namespace syncer {
 
-PassiveModelWorker::PassiveModelWorker(WorkerLoopDestructionObserver* observer)
-    : ModelSafeWorker(observer) {}
+PassiveModelWorker::PassiveModelWorker() = default;
 
 PassiveModelWorker::~PassiveModelWorker() {}
 
-void PassiveModelWorker::RegisterForLoopDestruction() {
-  SetWorkingLoopToCurrent();
-}
-
 SyncerError PassiveModelWorker::DoWorkAndWaitUntilDoneImpl(
     const WorkCallback& work) {
   // Simply do the work on the current thread.