[polymer] - Sort series data before adding to legend.
[polymer] - Sort series data before adding to legend.

Demo deployed: https://clean-simonhatch-1dffaf54-dot-chromeperf.appspot.com/group_report?keys=agxzfmNocm9tZXBlcmZyFAsSB0Fub21hbHkYgICghs24qQkM

R=aiolos@chromium.org
BUG=catapult:#2558

Review-Url: https://codereview.chromium.org/2233763003

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