De-Client CommandBuffer interface

De-Client CommandBuffer interface

This removes the CommandBufferClient interface from mojo.CommandBuffer.
It had two methods on it:

*) DidDestroy() which is redundant with the pipe closing, removed.
*) DidLoseContext(uint32 reason) which is used to signal an error code
prior to the pipe closing. Replaced with an observer interface.

BUG=451319
R=sky@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/fb327165a721c9279d478f982f6f41d83512e9fd

[jamesr, 2015-01-30 01:38:11]

jamesr@chromium.org changed reviewers:
+ sky@chromium.org

[jamesr, 2015-01-30 01:38:11]

(picking you somewhat arbitrarily as reviewer here, feel free to defer if you'd
like me to find someone else)

[sky, 2015-01-30 18:47:30]

LGTM

[jamesr, 2015-01-30 21:12:17]

Committed patchset #1 (id:1) manually as
fb327165a721c9279d478f982f6f41d83512e9fd (presubmit successful).