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:

Review URL: https://codereview.chromium.org/1286903003
1 file changed