Resolve FetchEvent.preloadResponse with undefined when Navigation Preload is disabled.

third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp

Index: third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp
diff --git a/third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp b/third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp
index 76dff738f3bb228eaf14be2968c90c770aa3338f..e5ed5d50a789b6fe23b99f6c95b5346ebaae8142 100644
--- a/third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp
+++ b/third_party/WebKit/Source/modules/serviceworkers/FetchEvent.cpp
@@ -76,11 +76,8 @@ FetchEvent::FetchEvent(ScriptState* scriptState,
           scriptState->getExecutionContext(),
           this,
           PreloadResponseProperty::PreloadResponse)) {
-  if (!navigationPreloadSent) {
-    // TODO(horo): This behavior is still under the spec discussion.
-    // https://github.com/w3c/ServiceWorker/issues/920#issuecomment-255874864
-    m_preloadResponseProperty->resolve(nullptr);
-  }
+  if (!navigationPreloadSent)
+    m_preloadResponseProperty->resolveWithUndefined();
 
   m_clientId = initializer.clientId();
   m_isReload = initializer.isReload();