Fix warning in string_utils.cpp in Win32 BUG= Change-Id: Ibb25d46c2ee5f361f6a948dc3efea38422a4573f Reviewed-on: https://chromium-review.googlesource.com/272559 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>