Fix webp compile warnings on windows

Fix webp compile warnings on windows

This fix was landed with:
https://codereview.chromium.org/1280073002/

The above CL was reverted due to an unrelated bug in libwebp.

The above CL contains multiple components, and I think that
reverting this part of the change was unintentional.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/340f3074b49de089e7e8cf9495fd54f67b7f9b8a

[msarett, 2015-08-17 13:55:48]

msarett@google.com changed reviewers:
+ djsollen@google.com, noel@chromium.org, scroggo@google.com

[msarett, 2015-08-17 13:55:49]

Noel, let me know if this fixes the compile issues that you've run into.

[djsollen, 2015-08-17 14:05:12]

lgtm

[msarett, 2015-08-17 14:43:11]

The CQ bit was checked by msarett@google.com

[commit-bot: I haz the power, 2015-08-17 14:43:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1286903003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1286903003/1

[commit-bot: I haz the power, 2015-08-17 14:49:40]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/340f3074b49de089e7e8cf9495fd54f67b7f9b8a

[Noel Gordon, 2015-08-18 03:03:20]

On 2015/08/17 13:55:49, msarett wrote:
> Noel, let me know if this fixes the compile issues that you've run into.

Yes that fixed the webp compile issue on WIN32, thank you.

LGTM.