Mark SimpleShaper members fields as private

Mark all the member fields in SimpleShaper as private.

BUG=418570
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/612173002

git-svn-id: svn://svn.chromium.org/blink/trunk@182869 bbb929c8-8fbe-4397-9dbb-9b2b20218538