[Polymer10] Upgrade base_chart.html to extend HTMLDivElement
[Polymer10] Upgrade base_chart.html to extend HTMLDivElement

HTMLUnknownElement is a really crazy thing to extend, so this changes
base_chart to extend HTMLDivElement and fixes 34 unit tests in the process.

BUG=catapult:#2285
R=aiolos@chromium.org

Review URL: https://codereview.chromium.org/1926103002 .

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