Cleanup: rename usage_mask --> usages.

Cleanup: rename usage_mask --> usages.

This is a simple search and replace:
  s/usage_mask/usages/g

There was some inconsistenty on when variables were called "usages" versus "usage_mask"; now they all read "usages".

Committed: https://crrev.com/0e1d34edba6a5d8f7fe43c5b675880a36f4b3861
Cr-Commit-Position: refs/heads/master@{#300533}

[eroman, 2014-10-21 18:08:41]

eroman@chromium.org changed reviewers:
+ rsleevi@chromium.org

[Ryan Sleevi, 2014-10-21 18:13:46]

lgtm

[eroman, 2014-10-21 18:15:34]

The CQ bit was checked by eroman@chromium.org

[commit-bot: I haz the power, 2014-10-21 18:17:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/670773003/1

[commit-bot: I haz the power, 2014-10-21 19:13:46]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-21 19:14:30]

Patchset 1 (id:??) landed as
https://crrev.com/0e1d34edba6a5d8f7fe43c5b675880a36f4b3861
Cr-Commit-Position: refs/heads/master@{#300533}