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

third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp

Index: third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp
diff --git a/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp b/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp
index a72902bd042852bb5c1211c073b05fe75f8f0821..73f3628792af66d6d7a8e3455041abb775c39086 100644
--- a/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp
+++ b/third_party/WebKit/Source/core/loader/WorkerThreadableLoader.cpp
@@ -668,6 +668,8 @@ void WorkerThreadableLoader::MainThreadLoaderHolder::start(
   DCHECK(isMainThread());
   ResourceLoaderOptions resourceLoaderOptions = originalResourceLoaderOptions;
   resourceLoaderOptions.requestInitiatorContext = WorkerContext;
+  // TODO(yhirano): Remove this CHECK once https://crbug.com/667254 is fixed.
+  CHECK(!m_mainThreadLoader);
   m_mainThreadLoader = DocumentThreadableLoader::create(document, this, options,
                                                         resourceLoaderOptions);
   m_mainThreadLoader->start(ResourceRequest(request.get()));