ipc: Make MessageAttachment inherit from RefCountedThreadSafe.

ipc: Make MessageAttachment inherit from RefCountedThreadSafe.

Previously, it inherited from RefCounted, which was wrong, since it's possible
for multiple threads to simultaneously have scoped_ptr references to a single
MessageAttachment.

BUG=535711
Committed: https://crrev.com/fa705362defe8ba29f7ba93570ea5cb8a5df0981
Cr-Commit-Position: refs/heads/master@{#357216}

[erikchen, 2015-10-30 21:25:17]

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

[erikchen, 2015-10-30 21:25:17]

tsepez: Please review.

[Tom Sepez, 2015-10-30 21:31:23]

lgtm

[erikchen, 2015-10-30 21:38:35]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-10-30 21:39:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1408793008/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1408793008/1

[commit-bot: I haz the power, 2015-10-30 23:16:36]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-30 23:17:21]

Patchset 1 (id:??) landed as
https://crrev.com/fa705362defe8ba29f7ba93570ea5cb8a5df0981
Cr-Commit-Position: refs/heads/master@{#357216}