Allow XHR timeout attribute to be overridden after send(), per spec

Source/core/loader/DocumentThreadableLoader.h

Index: Source/core/loader/DocumentThreadableLoader.h
diff --git a/Source/core/loader/DocumentThreadableLoader.h b/Source/core/loader/DocumentThreadableLoader.h
index 70a3322cf9450eda13b0975a09d9024ff5010fd6..3eb674fe8306e1bc36ed001a20732fc4b92d2cea 100644
--- a/Source/core/loader/DocumentThreadableLoader.h
+++ b/Source/core/loader/DocumentThreadableLoader.h
@@ -58,6 +58,8 @@ class DocumentThreadableLoader FINAL : public ThreadableLoader, private Resource
         static PassRefPtr<DocumentThreadableLoader> create(Document&, ThreadableLoaderClient*, const ResourceRequest&, const ThreadableLoaderOptions&);
         virtual ~DocumentThreadableLoader();
 
+        virtual void overrideTimeout(unsigned long timeout) OVERRIDE;
+
         virtual void cancel() OVERRIDE;
         void setDefersLoading(bool);
 
@@ -111,6 +113,7 @@ class DocumentThreadableLoader FINAL : public ThreadableLoader, private Resource
         OwnPtr<ResourceRequest> m_actualRequest; // non-null during Access Control preflight checks
         HTTPHeaderMap m_simpleRequestHeaders; // stores simple request headers in case of a cross-origin redirect.
         Timer<DocumentThreadableLoader> m_timeoutTimer;
+        double m_requestStartedSeconds; // Time an asynchronous fetch request is started
     };
 
 } // namespace WebCore