fix(benchpress): support measuring scriptTime and other metrics of page reload.

E.g. for benchmarks that measure page start time

modules/@angular/benchpress/test/metric/perflog_metric_spec.ts

@@ -171,6 +171,22 @@ export function main() {
            });
          }));
 
+      it('should mark and aggregate events since navigationStart',
+         inject([AsyncTestCompleter], (async: AsyncTestCompleter) => {
+           var events = [[
+             eventFactory.markStart('benchpress0', 0), eventFactory.start('script', 4),
+             eventFactory.end('script', 6), eventFactory.instant('navigationStart', 7),
+             eventFactory.start('script', 8), eventFactory.end('script', 9),
+             eventFactory.markEnd('benchpress0', 10)
+           ]];
+           var metric = createMetric(events, null);
+           metric.beginMeasure().then((_) => metric.endMeasure(false)).then((data) => {
+             expect(data['scriptTime']).toBe(1);
+
+             async.done();
+           });
+         }));
+
       it('should restart timing', inject([AsyncTestCompleter], (async: AsyncTestCompleter) => {
            var events = [
              [