Add an async ReadBody method to QuicChromiumClientStream::Handle

This does not change OnClose or OnError and the callback is not yet invoked
on errors. That will happen after all the upcalls are replaced by async
methods.

BUG=716563

Review-Url: https://codereview.chromium.org/2877063002
Cr-Original-Commit-Position: refs/heads/master@{#475172}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 27da045892e94846096a6549a178b34ee4d8f9d3
9 files changed