Use downloadToFile option when XHR downloads a Blob

Source/core/xml/XMLHttpRequest.h

Index: Source/core/xml/XMLHttpRequest.h
diff --git a/Source/core/xml/XMLHttpRequest.h b/Source/core/xml/XMLHttpRequest.h
index ad3546d55fd71439db1ed5813fe0e1e5681be0c7..d010acc0296e79214664406789bbfb398285bb7e 100644
--- a/Source/core/xml/XMLHttpRequest.h
+++ b/Source/core/xml/XMLHttpRequest.h
@@ -158,6 +158,10 @@ private:
     virtual void didSendData(unsigned long long bytesSent, unsigned long long totalBytesToBeSent);
     virtual void didReceiveResponse(unsigned long identifier, const ResourceResponse&);
     virtual void didReceiveData(const char* data, int dataLength);
+
+    // When "blob" is specified as the responseType attribute, didDownloadData
+    // is called instead of didReceiveData.
+    virtual void didDownloadData(int dataLength);
     virtual void didFinishLoading(unsigned long identifier, double finishTime);
     virtual void didFail(const ResourceError&);
     virtual void didFailRedirectCheck();