Require length in sk_wchar_to_string. This information is already available at all call sites and allows the call to WideCharToMultiByte to not overwrite the '\0' in the writable_str() which isn't really writable. BUG=skia:1989 R=reed@google.com Author: bungeman@google.com Review URL: https://codereview.chromium.org/500113002