fix(xhr): fix #1072, should set scheduled flag to target

[JiaLiPassion]

fix #1072.

• it is better to set scheduled flag to target than XMLHttpRequest. set to XMLHttpRequest is not a bug, but is not a good way to set such kind of flag to global object.
• issue More tasks executed then were scheduled error when xhrrequest a invalid url. #530, when open an invalid request, current logic will leave a scheduled macroTask in chrome. we should invoke the task to make the macroTask finish.

 var req = new XMLHttpRequest();
 req.open('get', 'file:///test', true);
 req.send();

[mhevery]

Needs rebase.

[JiaLiPassion]

@mhevery , got it, I will rebase it.