index.js

const puppeteer = require('puppeteer');
let page;

async function getBrowserPage() {
  // Launch headless Chrome. Turn off sandbox so Chrome can run under root.
  const browser = await puppeteer.launch({args: [
    '--no-sandbox'
  ]});
  return browser.newPage();
}

exports.screenshot_example = async (req, res) => {
  if (!page) {
    page = await getBrowserPage();
  }
  
  function scrapeForConsoleMessage(message) {
    return new Promise(function consoleListener(resolve, err) {
      page.on('console', msg => {
        // Turn on debugging with &debug in the request URL
        // All unity output is then logged in stackdriver
        if (req.query.debug) console.log(msg.text());
        if (msg.text() === message) {
          page.removeListener('console', consoleListener);
          resolve();
        }
      });    
    })
  }
  
  url = `https://storage.googleapis.com/public_website/examples/unity-serverless/build/index.html` + 
    `?r=${req.query.r || 0}&g=${req.query.g || 0}&b=${req.query.b || 0}`

  var scraper = scrapeForConsoleMessage('Screenshotter: capture now');
  console.log(`Fetching ${url}`);
  page.setViewport({
    width: 960,
    height: 600
  });
  page.goto(url);
  await scraper;
  console.log("Cloud function: Screenshotting")
  const gameContainerDOM = await page.$('#gameContainer');
  const imageBuffer = await gameContainerDOM.screenshot();
  res.set('Content-Type', 'image/png');
  res.send(imageBuffer);
};

Snapshotter.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Screenshotter : MonoBehaviour {
	public TextMesh text;
	public void Awake() {
		float r = 1, g = 1, b = 1;

		// Read r, g and b parameters passed into the query as <URL>?r=0.5?g=0?b=0.1
		string [] url_query = Application.absoluteURL.Split(new char[]{'?'}); 
		if (url_query.Length == 2) {
			var query = url_query[1];
			string [] assignments = query.Split(new char[]{'&'});
			foreach(string assignment in assignments) {
				string [] pair = assignment.Split(new char[]{'='});
				string lhs = WWW.UnEscapeURL(pair[0]);
				float rhs = float.Parse(WWW.UnEscapeURL(pair[1]));
				if (lhs == "r") r = rhs;
				else if (lhs == "g") g = rhs;
				else if (lhs == "b") b = rhs;
			}
		}
		
		// Color the text according to instruction in URL
		text.color = new Color(r, g, b, 1);

		// Wait for a frame to render
        StartCoroutine("snapshot");
    }

    IEnumerator snapshot() {
        yield return new WaitForEndOfFrame();
        Debug.Log("Screenshotter: capture now");
    }
}