Fix a sign-compare error to make gcc 4.9.2 happy.

Fix a sign-compare error to make gcc 4.9.2 happy.

This is from
https://chromium.googlesource.com/v8/v8/+/347fa90626a448e3535cf6aa100124dfd5b711ce

BUG=

Committed: https://crrev.com/145a2f45f3379029d6255a073e3eb762549ea47e
Cr-Commit-Position: refs/heads/master@{#30932}

[paul.l..., 2015-09-25 01:27:13]

The CQ bit was checked by paul.lind@imgtec.com to run a CQ dry run

[commit-bot: I haz the power, 2015-09-25 01:27:23]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1365113003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1365113003/1

[commit-bot: I haz the power, 2015-09-25 01:55:09]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-25 01:55:10]

Dry run: This issue passed the CQ dry run.

[paul.l..., 2015-09-25 01:57:34]

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

[paul.l..., 2015-09-25 01:57:35]

PTAL. If it looks OK, please commit it, as I won't be back online for several
more hours.

[oth, 2015-09-25 08:59:01]

On 2015/09/25 01:57:35, paul.l... wrote:
> PTAL. If it looks OK, please commit it, as I won't be back online for several
> more hours.

LGTM. Thanks Paul.

[rmcilroy, 2015-09-25 10:36:54]

LGTM.

[rmcilroy, 2015-09-25 10:49:40]

The CQ bit was checked by rmcilroy@chromium.org

[commit-bot: I haz the power, 2015-09-25 10:49:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1365113003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1365113003/1

[commit-bot: I haz the power, 2015-09-25 11:31:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-25 11:31:53]

Patchset 1 (id:??) landed as
https://crrev.com/145a2f45f3379029d6255a073e3eb762549ea47e
Cr-Commit-Position: refs/heads/master@{#30932}