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

chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.cc
index 43fcb86539a2539890171caedb8a75a610468367..149dfff57d1e70139f657b4f752961bb7ac9e426 100644
--- a/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/observers/service_worker_page_load_metrics_observer.cc
@@ -9,6 +9,11 @@
 
 namespace internal {
 
+const char kHistogramServiceWorkerParseStart[] =
+    "PageLoad.Clients.ServiceWorker.ParseTiming.NavigationToParseStart";
+const char kBackgroundHistogramServiceWorkerParseStart[] =
+    "PageLoad.Clients.ServiceWorker.ParseTiming.NavigationToParseStart."
+    "Background";
 const char kHistogramServiceWorkerFirstContentfulPaint[] =
     "PageLoad.Clients.ServiceWorker.PaintTiming."
     "NavigationToFirstContentfulPaint";
@@ -116,3 +121,18 @@ void ServiceWorkerPageLoadMetricsObserver::OnLoadEventStart(
                         timing.load_event_start.value());
   }
 }
+
+void ServiceWorkerPageLoadMetricsObserver::OnParseStart(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& info) {
+  if (!IsServiceWorkerControlled(info))
+    return;
+  if (WasStartedInForegroundOptionalEventInForeground(timing.parse_start,
+                                                      info)) {
+    PAGE_LOAD_HISTOGRAM(internal::kHistogramServiceWorkerParseStart,
+                        timing.parse_start.value());
+  } else {
+    PAGE_LOAD_HISTOGRAM(internal::kBackgroundHistogramServiceWorkerParseStart,
+                        timing.parse_start.value());
+  }
+}