Support chart.destroy()

[jwildfire]

webcharts is being updated with a chart.destroy() method. Lets add support for that functionality here as well.

Test notes

• No feature testing required. destroy() is a webcharts method that in this renderer also destroys the listing.

[samussiah]

safetyHistogram still returns a Webcharts object, so it's got a destroy() method. We'd also want to destroy the listing, so that should be done in the onDestroy() callback.

[dschwentker]

No testing needed, per Spencer.