chromedp.EvaluateAsDevTools("const paintTimings = performance.getEntriesByType('paint');", nil),
chromedp.EvaluateAsDevTools("const fmp = paintTimings.find(({ name }) => name === \"first-contentful-paint\");", nil),
chromedp.EvaluateAsDevTools("console.log('First contentful paint at foo');", &jscript))

package main
import (
	"context"
	"encoding/json"
	"log"
	"github.com/chromedp/chromedp"
)
type performancePaintTiming struct {
	EntryType string  `json:"entryType"`
	Name      string  `json:"name"`
	Duration  int     `json:"duration"`
	StartTime float64 `json:"startTime"`
}
func main() {
	ctx, cancel := chromedp.NewContext(context.Background(),
		// Enable the debug mode to see the CDP messages.
		// It's helpful to understand how CDP works.
		// But please don't enable it in Production Environment.
		chromedp.WithDebugf(log.Printf))
	defer cancel()
	// I don't know why but it seems that Runtime.evaluate does not return
	// JSON object any more, so I have to stringified it into a string.
	js := `
const paintTimings = performance.getEntriesByType('paint');
const fcp = paintTimings.find(({ name }) => name === "first-contentful-paint");
JSON.stringify(fcp);`
	var res string
	if err := chromedp.Run(ctx,
		chromedp.Navigate("https://www.bing.com/"),
		chromedp.EvaluateAsDevTools(js, &res),
	); err != nil {
		panic(err)
	}
	var fcp performancePaintTiming
	if err := json.Unmarshal([]byte(res), &fcp); err != nil {
		panic(err)
	}
	log.Printf("%#v", fcp)
}