Revert of Remove Client= from device/serial/data_stream.mojom.

Revert of Remove Client= from device/serial/data_stream.mojom. (patchset #1 id:20001 of https://codereview.chromium.org/889283002/)

Reason for revert:
Switching to two pipes broke FIFO assumption, causing DataSinkTest.CancelDuringError and SerialConnectionTest.Cancel to become flaky

Original issue's description:
> Remove Client= from device/serial/data_stream.mojom.
>
> BUG=451321
>
> Committed: https://crrev.com/bc78a2320b50eba4fe397f1f838a82ad30b16cd6
> Cr-Commit-Position: refs/heads/master@{#314231}

TBR=rockot@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=451321
Committed: https://crrev.com/4b7484b40acaec1d7904c5f38bd4be116468013a
Cr-Commit-Position: refs/heads/master@{#314524}

[Sam McNally, 2015-02-04 08:52:24]

Created Revert of Remove Client= from device/serial/data_stream.mojom.

[commit-bot: I haz the power, 2015-02-04 08:53:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/873293006/1

[commit-bot: I haz the power, 2015-02-04 08:54:24]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-04 08:55:05]

Patchset 1 (id:??) landed as
https://crrev.com/4b7484b40acaec1d7904c5f38bd4be116468013a
Cr-Commit-Position: refs/heads/master@{#314524}