[polymer] - Set "selected" field on seriesGroupList test in onCheckboxClicked().
[polymer] - Set "selected" field on seriesGroupList test in onCheckboxClicked().

Demo deployed: https://clean-simonhatch-0fab6c14-dot-chromeperf.appspot.com/group_report?keys=agxzfmNocm9tZXBlcmZyFAsSB0Fub21hbHkYgICg1tCXuAkM

BUG=catapult:#2621

Review-Url: https://codereview.chromium.org/2227563004

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