fail silently if CSSCoverage API is already enabled (projectwallace#21)

Author: bartveneman

_chromium.js

@@ -42,7 +42,8 @@ exports.extractCss = async url => {
 	const page = await getPage()
 
 	// Start CSS coverage. This is the meat and bones of this module
-	await page.coverage.startCSSCoverage()
+	// Fail silently if it's already enabled
+	await page.coverage.startCSSCoverage().catch(() => {})
 
 	const response = await page.goto(url, {waitUntil: 'networkidle2'})