Fix compilation of Uniscribe code path

Change the Windows Uniscribe code to use nullptr instead of 0 thereby
making it compile again.

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/178363007

git-svn-id: svn://svn.chromium.org/blink/trunk@167833 bbb929c8-8fbe-4397-9dbb-9b2b20218538