Add an async ReadInitialHeaders 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/2873963003
Cr-Commit-Position: refs/heads/master@{#473458}
9 files changed