[Sync] Replace uses of ObserverListThreadSafe with WeakHandles

chrome/browser/sync/js/js_mutation_event_observer.cc

Index: chrome/browser/sync/js/js_mutation_event_observer.cc
diff --git a/chrome/browser/sync/js/js_mutation_event_observer.cc b/chrome/browser/sync/js/js_mutation_event_observer.cc
index 5087a23ad0c12548f29efecca3cfe3a4e8053b15..2c323d1e054028941ec11b5ba9d4f1a81a397721 100644
--- a/chrome/browser/sync/js/js_mutation_event_observer.cc
+++ b/chrome/browser/sync/js/js_mutation_event_observer.cc
@@ -15,12 +15,21 @@
 
 namespace browser_sync {
 
-JsMutationEventObserver::JsMutationEventObserver() {}
+JsMutationEventObserver::JsMutationEventObserver()
+    : weak_ptr_factory_(ALLOW_THIS_IN_INITIALIZER_LIST(this)) {}
 
 JsMutationEventObserver::~JsMutationEventObserver() {
   DCHECK(non_thread_safe_.CalledOnValidThread());
 }
 
+base::WeakPtr<JsMutationEventObserver> JsMutationEventObserver::AsWeakPtr() {
+  return weak_ptr_factory_.GetWeakPtr();
+}
+
+void JsMutationEventObserver::InvalidateWeakPtrs() {
+  weak_ptr_factory_.InvalidateWeakPtrs();
+}
+
 void JsMutationEventObserver::SetJsEventHandler(
     const WeakHandle<JsEventHandler>& event_handler) {
   event_handler_ = event_handler;