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

third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp

Index: third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp
diff --git a/third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp b/third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp
index 70361e9f2e71b6129b42f1d70d40605dbc644787..a1e899cba22bf403538c6cda36f378ec0d4d3305 100644
--- a/third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp
+++ b/third_party/WebKit/Source/core/fileapi/FileReaderLoader.cpp
@@ -114,6 +114,8 @@ void FileReaderLoader::start(ExecutionContext* executionContext,
   ResourceLoaderOptions resourceLoaderOptions;
   resourceLoaderOptions.allowCredentials = AllowStoredCredentials;
 
+  // TODO(yhirano): Remove this CHECK once https://crbug.com/667254 is fixed.
+  CHECK(!m_loader);
   if (m_client) {
     DCHECK(!m_loader);
     m_loader = ThreadableLoader::create(*executionContext, this, options,