Add cstdarg header to log-utils.h

Add cstdarg header to log-utils.h

The log-utils.h file uses va_list but doesn't require the header. This CL
adds the needed header to remove a compiler error we've seen when doing some
bisecting.

Committed: https://crrev.com/e89226a549d73529aff5ed05d84e44bc1945b347
Cr-Commit-Position: refs/heads/master@{#31110}

[dsinclair, 2015-10-01 20:16:35]

dsinclair@chromium.org changed reviewers:
+ adamk@chromium.org

[dsinclair, 2015-10-01 20:16:35]

PTAL.

[adamk, 2015-10-05 17:44:51]

The CQ bit was checked by adamk@chromium.org

[adamk, 2015-10-05 17:44:51]

lgtm

[commit-bot: I haz the power, 2015-10-05 17:45:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1383483004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1383483004/1

[commit-bot: I haz the power, 2015-10-05 18:54:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-05 18:54:18]

Patchset 1 (id:??) landed as
https://crrev.com/e89226a549d73529aff5ed05d84e44bc1945b347
Cr-Commit-Position: refs/heads/master@{#31110}