fix CSP by injecting styles using CSSOM

This uses CSS Object Model (CSSOM) to modify stylesheets.
Changing stylesheets via CSSOM does not violate
Content-Security-Policy style-src 'none'.

CSSOM is still a working draft but the features been used should
be supported by all target browsers:
https://developer.mozilla.org/en-US/docs/Web/API/DocumentOrShadowRoot/styleSheets#Browser_compatibility
https://developer.mozilla.org/en-US/docs/Web/API/CSSStyleSheet/insertRule#Browser_compatibility

Creating an empty style element does not violate CSP.

-webkit- prefix is not needed anymore for keyframe.
Inserting CSS rule @-webkit-keyframews throws a SyntaxError in IE11.

Done basic manual testing using samples in recent versions of:
- Chrome (Desktop & Mobile)
- Firefox
- Microsoft Edge
- IE 11

Fixes chartjs#5208 together with chartjs#5909

Live example: https://codepen.io/jelhan/pen/jXYymO

Please note the CSP meta tag definied in settings. You need to update SHA
hashes if you change any JavaScript in this Codepen as it violates CSP
otherwise.

src/platforms/platform.dom.js

@@ -304,17 +304,19 @@ function removeResizeListener(node) {
 	}
 }
 
-function injectCSS(platform, css) {
-	// https://stackoverflow.com/q/3922139
-	var style = platform._style || document.createElement('style');
-	if (!platform._style) {
-		platform._style = style;
-		css = '/* Chart.js */\n' + css;
-		style.setAttribute('type', 'text/css');
-		document.getElementsByTagName('head')[0].appendChild(style);
+function injectCSS(platform, cssRules) {
+	if (!platform._stylesheet) {
+		var styleElement = document.createElement('style');
+		styleElement.setAttribute('type', 'text/css');
+		document.getElementsByTagName('head')[0].appendChild(styleElement);
+
+		platform._stylesheet = styleElement.sheet;
 	}
 
-	style.appendChild(document.createTextNode(css));
+	cssRules.forEach(function(cssRule) {
+		// index is required for IE11
+		platform._stylesheet.insertRule(cssRule, 0);
+	});
 }
 
 module.exports = {
@@ -328,16 +330,15 @@ module.exports = {
 	initialize: function() {
 		var keyframes = 'from{opacity:0.99}to{opacity:1}';
 
-		injectCSS(this,
+		injectCSS(this, [
 			// DOM rendering detection
 			// https://davidwalsh.name/detect-node-insertion
-			'@-webkit-keyframes ' + CSS_RENDER_ANIMATION + '{' + keyframes + '}' +
-			'@keyframes ' + CSS_RENDER_ANIMATION + '{' + keyframes + '}' +
+			'@keyframes ' + CSS_RENDER_ANIMATION + '{' + keyframes + '}',
 			'.' + CSS_RENDER_MONITOR + '{' +
 				'-webkit-animation:' + CSS_RENDER_ANIMATION + ' 0.001s;' +
 				'animation:' + CSS_RENDER_ANIMATION + ' 0.001s;' +
 			'}'
-		);
+		]);
 	},
 
 	acquireContext: function(item, config) {