Enable running on node

[SeanSobey]

A pretty small change, which allows ChartJS to run on Node using a virtual canvas like node-canvas.

All that is required is that browser specific APIs are not referenced, like window or document. I only found 1 that was missing an undefined guard.

ChartJS can be used on node like so:

const ChartJs = require('chart.js');
const { createCanvas } = require('canvas');

function renderPNG(configuration) {
	
	const canvas = createCanvas(800, 600);	
	// Disable animation (otherwise charts will throw exceptions)
	configuration.options = configuration.options || {};
	configuration.options.responsive = false;
	configuration.options.animation = false;
	canvas.style = {};
	const context = canvas.getContext('2d');
	const chart = new ChartJs(context, configuration);
	return new Promise((resolve, reject) => {
                // or `pngStream` `toDataURL`, etc
		chart.canvas.toBuffer((error, buffer) => {
			if (error) {
				return reject(error);
			}
			return resolve(buffer);
		});
	});
}