Fix a -Wsign-compare error under GCC 4.9.2.

Fix a -Wsign-compare error under GCC 4.9.2.

BUG=

Committed: https://crrev.com/0eacd754cc11d6f658703b5240af88a6b14b5d2d
Cr-Commit-Position: refs/heads/master@{#29795}

[paul.l..., 2015-07-23 04:48:23]

paul.lind@imgtec.com changed reviewers:
+ jkummerow@chromium.org, yangguo@chromium.org

[paul.l..., 2015-07-23 04:48:24]

PTAL. If this looks ok, please commit it as well (I won't be back online for 8
or more hours, and our target bots are whining about this :)

[Yang, 2015-07-23 06:00:43]

The CQ bit was checked by yangguo@chromium.org

[Yang, 2015-07-23 06:00:44]

lgtm

[commit-bot: I haz the power, 2015-07-23 06:00:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1249703005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1249703005/1

[commit-bot: I haz the power, 2015-07-23 06:27:10]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-23 06:27:26]

Patchset 1 (id:??) landed as
https://crrev.com/0eacd754cc11d6f658703b5240af88a6b14b5d2d
Cr-Commit-Position: refs/heads/master@{#29795}