Change SimpleShaper::advance to take unsigned

Change the SimpleShaper::advance signature and implementation to take an
unsigned thereby clarifying the API and avoiding an unnecessary (and
potentially unsafe) cast.

R=pdr@chromium.org
BUG=418570

Review URL: https://codereview.chromium.org/608413002

git-svn-id: svn://svn.chromium.org/blink/trunk@182871 bbb929c8-8fbe-4397-9dbb-9b2b20218538