Fix race in HostSharedBitmapManager::AllocatedBitmapCount

Fix race in HostSharedBitmapManager::AllocatedBitmapCount

This race should be benign, because the count is just used for a heuristic and can't truly be synchronized anyway, but best to use a lock with the access.

BUG=417194
Committed: https://crrev.com/b86d19a65aa6e002f7421a724f5fe29f62a016ae
Cr-Commit-Position: refs/heads/master@{#296554}

[jbauman, 2014-09-24 21:09:20]

jbauman@chromium.org changed reviewers:
+ piman@chromium.org

[piman, 2014-09-24 21:55:45]

lgtm

[jbauman, 2014-09-24 21:55:55]

The CQ bit was checked by jbauman@chromium.org

[commit-bot: I haz the power, 2014-09-24 21:57:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/604583002/1

[commit-bot: I haz the power, 2014-09-24 23:07:03]

Committed patchset #1 (id:1) as 497095694a5024680e7b8c89b3d3049d99a5f2c3

[commit-bot: I haz the power, 2014-09-24 23:07:38]

Patchset 1 (id:??) landed as
https://crrev.com/b86d19a65aa6e002f7421a724f5fe29f62a016ae
Cr-Commit-Position: refs/heads/master@{#296554}