Fix ServiceWorker bg paint metric.

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 97bc2d210ad948ac3b5b6c792947c206485879c2..8a47c21ffa8db75827819f1f448abd71aec435d1 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
@@ -34,14 +34,16 @@ void ServiceWorkerPageLoadMetricsObserver::OnComplete(
 void ServiceWorkerPageLoadMetricsObserver::LogServiceWorkerHistograms(
     const page_load_metrics::PageLoadTiming& timing,
     const page_load_metrics::PageLoadExtraInfo& info) {
-  bool foreground_paint = WasStartedInForegroundEventInForeground(
-      timing.first_contentful_paint, info);
-  if (foreground_paint) {
-    PAGE_LOAD_HISTOGRAM(internal::kHistogramServiceWorkerFirstContentfulPaint,
-                        timing.first_contentful_paint);
-  } else {
-    PAGE_LOAD_HISTOGRAM(
-        internal::kBackgroundHistogramServiceWorkerFirstContentfulPaint,
-        timing.first_contentful_paint);
+  if (!timing.first_contentful_paint.is_zero()) {
+    bool foreground_paint = WasStartedInForegroundEventInForeground(
+        timing.first_contentful_paint, info);
+    if (foreground_paint) {
+      PAGE_LOAD_HISTOGRAM(internal::kHistogramServiceWorkerFirstContentfulPaint,
+                          timing.first_contentful_paint);
+    } else {
+      PAGE_LOAD_HISTOGRAM(
+          internal::kBackgroundHistogramServiceWorkerFirstContentfulPaint,
+          timing.first_contentful_paint);
+    }
   }
 }