puppeteer: Add screenshot method to common module.

This will be useful for debugging purposes and we'll use it to take
a screenshot on failure.

frontend_tests/puppeteer_lib/common.js

@@ -1,8 +1,10 @@
+const path = require('path');
 const puppeteer = require('puppeteer');
 
 class CommonUtils {
     constructor() {
         this.browser = null;
+        this.screenshot_id = 0;
     }
 
     async ensure_browser() {
@@ -29,6 +31,19 @@ class CommonUtils {
         return page;
     }
 
+    async screenshot(page, name = null) {
+        if (name === null) {
+            name = `${this.screenshot_id}`;
+            this.screenshot_id += 1;
+        }
+
+        const root_dir = path.resolve(__dirname, '../../');
+        const screenshot_path = path.join(root_dir, 'var/puppeteer', `${name}.png`);
+        await page.screenshot({
+            path: screenshot_path,
+        });
+    }
+
     async run_test(test_function) {
         try {
             await test_function();