Add Request::GetStreamData() method

This method is going to be used to read streaming request data
asynchronously and will replace Request::GetData().

BUG: 24166746
Change-Id: Id36106cf59725044b71ed83695e31b67ae60ca05
Reviewed-on: https://weave-review.googlesource.com/1098
Reviewed-by: Vitaly Buka <vitalybuka@google.com>