BackgroundSync sync events need to be ExtendableEvents, blink side

Source/modules/serviceworkers/WaitUntilObserver.cpp

Index: Source/modules/serviceworkers/WaitUntilObserver.cpp
diff --git a/Source/modules/serviceworkers/WaitUntilObserver.cpp b/Source/modules/serviceworkers/WaitUntilObserver.cpp
index d652b7cb0539bab95664dd15a8104552f0e50d8d..4c15898a06d36bf71dc8313462ed83cda4587d85 100644
--- a/Source/modules/serviceworkers/WaitUntilObserver.cpp
+++ b/Source/modules/serviceworkers/WaitUntilObserver.cpp
@@ -178,6 +178,9 @@ void WaitUntilObserver::decrementPendingActivity()
     case Push:
         client->didHandlePushEvent(m_eventID, result);
         break;
+    case Sync:
+        client->didHandleSyncEvent(m_eventID, result);
+        break;
     }
     setContext(nullptr);
 }