Added 'angle' option to Polar Charts (#5279)

* added 'angle' option to polar charts. image comparison test is work in progress; not currently passing

* removed unnecessary variable assignment

* code cleanup based on PR; for 'angle' option on polarCharts

* Made polar chart image comparison test pass by removing debug flag. Also explicitly marked _computeAngle as private.

* Removed visibleCount computation in polar chart

* split out code related to updating the radius in polar chart's update function, into it's own 'updateRadius' function

* made updateRadius method private

* fix linting error

* updated polar charts to read custom angles from "chart.options.elements.arc.angle" instead of "chart.options.angle"