Introduce DataPipeBytesConsumer::CompletionNotifier

Move SignalComplete and SignalError from DataPipeBytesConsumer to the
class to separate them from BytesConsumer interface.

Bug: 894815
Change-Id: Ib592ba7615c9c0d0fa24e9a9e1a7a07f56cca8ab
Reviewed-on: https://chromium-review.googlesource.com/c/1304115
Reviewed-by: Ben Kelly <wanderview@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604530}
5 files changed