XHR.abort(): no event dispatching in completed/non-started states.

Follow the spec and other implementations and only transition to an
UNSENT state if abort() is called on request that either hasn't been
sent or has completed. That is, no dispatching of progress events will
be done.

R=
BUG=315470

Review URL: https://codereview.chromium.org/60203010

git-svn-id: svn://svn.chromium.org/blink/trunk@161515 bbb929c8-8fbe-4397-9dbb-9b2b20218538