ipc: Add a lock to guard modifications of state of the AttachmentBroker.

ipc: Add a lock to guard modifications of state of the AttachmentBroker.

The AttachmentBroker can be accessed from any thread, so any modifications to
internal state should be guarded by a lock.

BUG=535711
Committed: https://crrev.com/282e55ac60a1c02c5e0ac82269ed42966aff8381
Cr-Commit-Position: refs/heads/master@{#356972}

[erikchen, 2015-10-29 20:40:27]

erikchen@chromium.org changed reviewers:
+ tsepez@chromium.org

[erikchen, 2015-10-29 20:40:27]

tsepez: Please review.

[Tom Sepez, 2015-10-29 20:45:44]

That certainly explains a lot. LGTM.

[erikchen, 2015-10-29 20:51:02]

On 2015/10/29 20:45:44, Tom Sepez wrote:
> That certainly explains a lot. LGTM.

This is a necessary change, but I haven't been able to prove that this is the
actual cause of the GPU conformance test failures (just another theory, and some
local debugging implies this probably isn't it).

[erikchen, 2015-10-29 20:51:06]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-10-29 20:52:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1405253010/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1405253010/1

[commit-bot: I haz the power, 2015-10-29 22:07:06]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-29 22:07:07]

Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[erikchen, 2015-10-29 22:08:25]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-10-29 22:09:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1405253010/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1405253010/1

[commit-bot: I haz the power, 2015-10-29 22:54:49]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-29 22:55:34]

Patchset 1 (id:??) landed as
https://crrev.com/282e55ac60a1c02c5e0ac82269ed42966aff8381
Cr-Commit-Position: refs/heads/master@{#356972}