fix(benchpress): work around missing events from Chrome 63 (#21396)

Chrome 63 can cause the navigationStart event for the first
run to arrive with a different pid than the start of the
benchpress run. This makes the first collected result invalid.

This workaround causes the sampler to ignore runs that have this
condition.

PR Close #21396

packages/benchpress/src/metric/perflog_metric.ts

@@ -249,6 +249,9 @@ export class PerflogMetric extends Metric {
       // not all events have been received, no further processing for now
       return null;
     }
+    if (markStartEvent.pid !== markEndEvent.pid) {
+      result['invalid'] = 1;
+    }
 
     let gcTimeInScript = 0;
     let renderTimeInScript = 0;

packages/benchpress/src/sampler.ts

@@ -63,7 +63,12 @@ export class Sampler {
     }
     return resultPromise.then((_) => this._driver.waitFor(this._execute))
         .then((_) => this._metric.endMeasure(this._prepare === Options.NO_PREPARE))
-        .then((measureValues) => this._report(lastState, measureValues));
+        .then((measureValues) => {
+          if (!!measureValues['invalid']) {
+            return lastState;
+          }
+          return this._report(lastState, measureValues);
+        });
   }
 
   private _report(state: SampleState, metricValues: {[key: string]: any}): Promise<SampleState> {