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
3 files changed