Avoid setTimeout in waitOnElement (WebKit#321)

waitOnElement uses a 50ms timeout, which was historically motivated by FlightJS that used requirejs which had a hard-coded 50ms pause.

We've observed fragile CPU scaling behavior with this approach in Speedometer 2.1. V3.0 is less affected by this since the perf-dashboard is the only workload the regularly needs another cycle to wait (all other workloads are ready after onload has fired).

Using rAF lowers the timeout limit and is more realistic in required CPU ramp-up performance.

resources/benchmark-runner.mjs

@@ -33,13 +33,10 @@ class Page {
         return new Promise((resolve) => {
             const resolveIfReady = () => {
                 const element = this.querySelector(selector);
-                if (element) {
-                    window.requestAnimationFrame(() => {
-                        return resolve(element);
-                    });
-                } else {
-                    setTimeout(resolveIfReady, 50);
-                }
+                let callback = resolveIfReady;
+                if (element)
+                    callback = () => resolve(element);
+                window.requestAnimationFrame(callback);
             };
             resolveIfReady();
         });