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

http://www.w3.org/TR/XMLHttpRequest/#the-timeout-attribute hints that this attribute may be overridden
after a request has been sent. The web-platform-tests suite has tests verifying that this is handled
correctly, and other browsers such as Gecko-based browsers are successfully passing them, while Blink
and WebKit are not. Therefore, there is an API-compat issue.

BUG=371639,387470,387569

Review URL: https://codereview.chromium.org/273993002

git-svn-id: svn://svn.chromium.org/blink/trunk@179730 bbb929c8-8fbe-4397-9dbb-9b2b20218538