puppeteer_lib: Use class fields.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

frontend_tests/puppeteer_lib/common.js

@@ -12,11 +12,10 @@ const root_dir = path.resolve(__dirname, "../../");
 const puppeteer_dir = path.join(root_dir, "var/puppeteer");
 
 class CommonUtils {
-    constructor() {
-        this.browser = null;
-        this.screenshot_id = 0;
-        this.realm_url = "http://zulip.zulipdev.com:9981/";
-        this.pm_recipient = {
+    browser = null;
+    screenshot_id = 0;
+    realm_url = "http://zulip.zulipdev.com:9981/";
+    pm_recipient = {
         async set(page, recipient) {
             // Without using the delay option here there seems to be
             // a flake where the typeahead doesn't show up.
@@ -47,17 +46,16 @@ class CommonUtils {
         },
     };
 
-        this.fullname = {
+    fullname = {
         cordelia: "Cordelia Lear",
         othello: "Othello, the Moor of Venice",
         hamlet: "King Hamlet",
     };
 
-        this.window_size = {
+    window_size = {
         width: 1400,
         height: 1024,
     };
-    }
 
     async ensure_browser() {
         if (this.browser === null) {