Add histograms for time to parse start for service worker controlled pages.

chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.h b/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.h
index c34fcbe6663e9c6922632298b0c58081ceb36721..d0e9c2f2599f9039242c0993d8581cb1f9324e6c 100644
--- a/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.h
@@ -11,6 +11,8 @@
 namespace internal {
 
 // Expose metrics for tests.
+extern const char kHistogramServiceWorkerParseStart[];
+extern const char kBackgroundHistogramServiceWorkerParseStart[];
 extern const char kHistogramServiceWorkerFirstContentfulPaint[];
 extern const char kBackgroundHistogramServiceWorkerFirstContentfulPaint[];
 extern const char kHistogramServiceWorkerParseStartToFirstContentfulPaint[];
@@ -29,6 +31,9 @@ class ServiceWorkerPageLoadMetricsObserver
  public:
   ServiceWorkerPageLoadMetricsObserver();
   // page_load_metrics::PageLoadMetricsObserver implementation:
+  void OnParseStart(
+      const page_load_metrics::PageLoadTiming& timing,
+      const page_load_metrics::PageLoadExtraInfo& extra_info) override;
   void OnFirstContentfulPaint(
       const page_load_metrics::PageLoadTiming& timing,
       const page_load_metrics::PageLoadExtraInfo& extra_info) override;