Fix drawPosText() bounds bug.

We didn't catch this in our local tests because we tend to use default
kUTF8_TextEncoding with single-byte characters, which means N == byteLength.

BUG=409110
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/531933002
2 files changed