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This is a reland of e9605182

The original CL added a unit-test which created a single-byte
Channel::Message, without actually initializing that single-byte. This
caused the MSAN bots to (correctly) spot that uninitialized data was
being read during serialization.

Original change's description:
> Delay Channel::OnError() in case of kDisconnected during Write().
>
> Write() operations to a Channel can fail due to the peer having closed
> it, while there are still messages waiting to be read from it. We must
> therefore defer notifying the caller of the Channel::Error until we
> observe end-of-stream via a readable notification, otherwise those
> messages may be dropped (depending on whether the posted OnError task
> is processed before or after a pending Channel-readable event).
>
> Bug: 816620
> Change-Id: I75bd34a48edf4022809d27ce49f9cfba7a5d4daf
> Reviewed-on: https://chromium-review.googlesource.com/956932
> Commit-Queue: Wez <wez@chromium.org>
> Reviewed-by: Ken Rockot <rockot@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#542634}

TBR: rockot
Bug: 816620
Change-Id: I1a1d6eb7fa712e50b3d9c86591878900f0aeb388
Reviewed-on: https://chromium-review.googlesource.com/959762Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542739}