Refactor tooltip draw function to extract drawBackground method

[mjeanroy]

See #3416

[etimberg]

+1 to merge.

[kasimkha]

I have done the changes in my core.tooltip.js file then generated bundle for the minified js files and using this in my angular module. For setting background of tooltip dynamically i have added
Chart.Tooltip.prototype.drawBackground = function(pt, vm, ctx, tooltipSize, opacity) {
// New implementation
debugger;
if (vm.dataPoints[0].xLabel == 2) {
vm.backgroundColor = 'rgb(45,156,219)';

}
return vm.backgroundColor;
}; but the same isn't seem to be working.Am i doing something wrong ?

[kasimkha]

https://stackoverflow.com/questions/51004178/is-there-a-way-to-customize-the-tooltip-backgroup-color-dynamically-in-chartjs - created issue on github for the same