[polymer] - Fix initial checkbox state.
[polymer] - Fix initial checkbox state.

Demo deployed: https://clean-simonhatch-0f569374-dot-chromeperf.appspot.com/group_report?keys=agxzfmNocm9tZXBlcmZyFAsSB0Fub21hbHkYgICglraCswoM,agxzfmNocm9tZXBlcmZyFAsSB0Fub21hbHkYgICglonYrwoM,agxzfmNocm9tZXBlcmZyFAsSB0Fub21hbHkYgICglvagrgoM

R=aiolos@chromium.org
BUG=catapult:#2655

Review-Url: https://codereview.chromium.org/2235243002

catapult-project/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,sahiljain/catapult,benschmaus/catapult,sahiljain/catapult,catapult-project/catapult-csm,sahiljain/catapult,benschmaus/catapult,benschmaus/catapult