Add CHECKs to make sure ThreadableLoader has not been created multiple times

third_party/WebKit/Source/web/WebAssociatedURLLoaderImpl.cpp

Index: third_party/WebKit/Source/web/WebAssociatedURLLoaderImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebAssociatedURLLoaderImpl.cpp b/third_party/WebKit/Source/web/WebAssociatedURLLoaderImpl.cpp
index 74a2490f58ba2130d83633c644796c39e578adfa..69a0e411edd78b3a571308f24d28fb990a26f504 100644
--- a/third_party/WebKit/Source/web/WebAssociatedURLLoaderImpl.cpp
+++ b/third_party/WebKit/Source/web/WebAssociatedURLLoaderImpl.cpp
@@ -410,6 +410,8 @@ void WebAssociatedURLLoaderImpl::loadAsynchronously(
 
     Document* document = toDocument(m_observer->lifecycleContext());
     DCHECK(document);
+    // TODO(yhirano): Remove this CHECK once https://crbug.com/667254 is fixed.
+    CHECK(!m_loader);
     m_loader = DocumentThreadableLoader::create(
         *document, m_clientAdapter.get(), options, resourceLoaderOptions);
     m_loader->start(webcoreRequest);