[polymer] - Fix start bisect button always disabled when form showed.
[polymer] - Fix start bisect button always disabled when form showed.

Demo deployed: https://clean-simonhatch-ae72aad9-dot-chromeperf.appspot.com/report?sid=c7a9a3f02497dff3a4c299e3f35e4b7a28bdf7c383e41f49a0231d79b3fe190c&rev=409569

R=sullivan@chromium.org
BUG=catapult:#2613

Review-Url: https://codereview.chromium.org/2211943002

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