XHR with responseType='arraybuffer' should not return null as response

When response entity body is null, XHR with responseType='arraybuffer'
should return an empty ArrayBuffer as xhr.response[1].

[1]: http://www.w3.org/TR/XMLHttpRequest2/#arraybuffer-response-entity-body

BUG=287697

Review URL: https://chromiumcodereview.appspot.com/23986004

git-svn-id: svn://svn.chromium.org/blink/trunk@157504 bbb929c8-8fbe-4397-9dbb-9b2b20218538