Make performance and position private since they don’t sync
Make performance and position private since they don’t sync

R=abarth@chromium.org, abarth

Review URL: https://codereview.chromium.org/1230273004 .

afandria/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,jianglu/mojo,afandria/mojo,jianglu/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,afandria/mojo