Synchronize on concurrent slot buffer entries during migration.

Synchronize on concurrent slot buffer entries during migration.

BUG=chromium:524425
LOG=n

Committed: https://crrev.com/7ab389a437696fda3263f393f9bbbfdae0c7e9e2
Cr-Commit-Position: refs/heads/master@{#30423}

[Hannes Payer (out of office), 2015-08-27 14:46:26]

hpayer@chromium.org changed reviewers:
+ mlippautz@chromium.org

[Michael Lippautz, 2015-08-27 15:07:03]

lgtm -- this may indeed be too slow at some point

[Hannes Payer (out of office), 2015-08-27 16:28:14]

The CQ bit was checked by hpayer@chromium.org

[Hannes Payer (out of office), 2015-08-27 16:28:15]

The patchset sent to the CQ was uploaded after l-g-t-m from
mlippautz@chromium.org
Link to the patchset: https://codereview.chromium.org/1314133004/#ps20001
(title: " ")

[commit-bot: I haz the power, 2015-08-27 16:28:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1314133004/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1314133004/20001

[commit-bot: I haz the power, 2015-08-27 16:54:10]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-08-27 16:54:22]

Patchset 2 (id:??) landed as
https://crrev.com/7ab389a437696fda3263f393f9bbbfdae0c7e9e2
Cr-Commit-Position: refs/heads/master@{#30423}