servo/servo — pull request #3732 by mukilan (2014-10-20 15:40:15+00:00)
Title: Fix race condition in XHR and handle other abort/open scenarios

This fixes issue #3630
A short summary of the changes:
* Use atomic generation id to cancel inflight requests
* Handles nested calls to abort, open, send inside handlers
* Adds XHRReleaseMsg to delay freeing XHR object till all
inflight events are processed
* Handles both timeout, errors and abort/open in a symmetric fashion
i.e All inflight events will be cancelled for timeouts, aborts,
errors and on calling open.
* Change the ErroredMsg enum to be more symmetric with the returned
Error enum

I noticed a few possible changes that could make the code for fetch task simpler:
* We can remove the additional timer task and let the fetch task manage
its own timer (or maybe the resource loader can do this.)
* The CORS related steps could also be moved into the resource loader.
* Right now upload events are not support. This requires some support
from resource loader.