[XHR] chunked requests

One use case that we have which is not currently handled by XMLHttpRequest
is incrementally sending data that takes a long time to generate _from the
client to the server_. For example, if we were to record data from a
microphone, we couldn't upload it in real time to the server with the
current API.

The MediaStreaming spec also mentioned several use cases which would
require streaming request data via an API:
- Sending the locally-produced streams to remote peers and receiving
streams from remote peers.
- Sending arbitrary data to remote peers.

http://www.whatwg.org/specs/web-apps/current-work/multipage/video-conferencing-and-peer-to-peer-communication.html

- Wenbo