Split the constructor of ThreadableLoader into two methods (ctor and start())

Source/modules/fetch/FetchManager.cpp

Index: Source/modules/fetch/FetchManager.cpp
diff --git a/Source/modules/fetch/FetchManager.cpp b/Source/modules/fetch/FetchManager.cpp
index e746bc53289498fa74207e17d4e5c6d82d427f3d..4ed2abce806c04539b902a9f959da31cc6c56c63 100644
--- a/Source/modules/fetch/FetchManager.cpp
+++ b/Source/modules/fetch/FetchManager.cpp
@@ -381,9 +381,8 @@ void FetchManager::Loader::performHTTPFetch(bool corsFlag, bool corsPreflightFla
         threadableLoaderOptions.crossOriginRequestPolicy = UseAccessControl;
         break;
     }
-    m_loader = ThreadableLoader::create(*executionContext(), this, request, threadableLoaderOptions, resourceLoaderOptions);
-    if (!m_loader)
-        performNetworkError("Can't create ThreadableLoader");
+    m_loader = ThreadableLoader::create(*executionContext(), this, threadableLoaderOptions, resourceLoaderOptions);
+    m_loader->start(request);
 }
 
 void FetchManager::Loader::failed(const String& message)