Reenable optimization on XMLHttpRequest's receive-as-blob functionality

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 1daf17eb1faeea5a1887d692052968391249981a..3c665913847a977c40137f39d266426cc98974bd 100644
--- a/Source/core/xml/XMLHttpRequest.h
+++ b/Source/core/xml/XMLHttpRequest.h
@@ -155,6 +155,9 @@ private:
     virtual void didSendData(unsigned long long bytesSent, unsigned long long totalBytesToBeSent) OVERRIDE;
     virtual void didReceiveResponse(unsigned long identifier, const ResourceResponse&) OVERRIDE;
     virtual void didReceiveData(const char* data, int dataLength) OVERRIDE;
+    // When responseType is set to "blob", didDownloadData() is called instead
+    // of didReceiveData().
+    virtual void didDownloadData(int dataLength) OVERRIDE;
     virtual void didFinishLoading(unsigned long identifier, double finishTime) OVERRIDE;
     virtual void didFail(const ResourceError&) OVERRIDE;
     virtual void didFailRedirectCheck() OVERRIDE;
@@ -233,6 +236,8 @@ private:
     RefPtrWillBeMember<Document> m_responseDocument;
 
     RefPtr<SharedBuffer> m_binaryResponseBuilder;
+    long long m_downloadedBlobLength;
+
     RefPtr<ArrayBuffer> m_responseArrayBuffer;
 
     bool m_error;