Make ChannelHandle::socket optional in mus' ChannelHandle mojom.

Make ChannelHandle::socket optional in mus' ChannelHandle mojom.

This case is already supported by the type converter, but a dchecks build
will fail with a validation error if this is ever the case. When GPU channels
use ChannelMojo, this will be the case 100% of the time.

BUG=604282
Committed: https://crrev.com/8fcab50ecd01fcde3d70b2b05fdeef528249af1c
Cr-Commit-Position: refs/heads/master@{#404078}

[Anand Mistry (off Chromium), 2016-07-07 03:23:36]

amistry@chromium.org changed reviewers:
+ dcheng@chromium.org, penghuang@chromium.org

[Anand Mistry (off Chromium), 2016-07-07 03:23:36]

See test failures in https://codereview.chromium.org/2127693002/ (i.e.
https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium...)
for the motivation behind this change.

[dcheng, 2016-07-07 03:25:18]

lgtm

[Peng, 2016-07-07 03:40:44]

On 2016/07/07 03:25:18, dcheng wrote:
> lgtm

lgtm

[Anand Mistry (off Chromium), 2016-07-07 03:41:33]

The CQ bit was checked by amistry@chromium.org

[commit-bot: I haz the power, 2016-07-07 03:41:50]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-07-07 04:56:16]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-07-07 04:56:18]

CQ bit was unchecked.

[commit-bot: I haz the power, 2016-07-07 04:58:01]

Description was changed from

==========
Make ChannelHandle::socket optional in mus' ChannelHandle mojom.

This case is already supported by the type converter, but a dchecks build
will fail with a validation error if this is ever the case. When GPU channels
use ChannelMojo, this will be the case 100% of the time.

BUG=604282
==========

to

==========
Make ChannelHandle::socket optional in mus' ChannelHandle mojom.

This case is already supported by the type converter, but a dchecks build
will fail with a validation error if this is ever the case. When GPU channels
use ChannelMojo, this will be the case 100% of the time.

BUG=604282

Committed: https://crrev.com/8fcab50ecd01fcde3d70b2b05fdeef528249af1c
Cr-Commit-Position: refs/heads/master@{#404078}
==========

[commit-bot: I haz the power, 2016-07-07 04:58:03]

Patchset 1 (id:??) landed as
https://crrev.com/8fcab50ecd01fcde3d70b2b05fdeef528249af1c
Cr-Commit-Position: refs/heads/master@{#404078}