Remove call to WasHidden as part of the app being backgrounded.

chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.cc
index 50fc1417bf68e062ae617a7bffdfe7759d93106c..50e1cdbaa1ab92012a541d2ad8bb643fb5b59d33 100644
--- a/chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.cc
@@ -132,6 +132,18 @@ DataReductionProxyMetricsObserver::OnHidden(
   return STOP_OBSERVING;
 }
 
+page_load_metrics::PageLoadMetricsObserver::ObservePolicy
+DataReductionProxyMetricsObserver::FlushMetricsOnAppEnterBackground(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& info) {
+  // FlushMetricsOnAppEnterBackground is invoked on Android in cases where the
+  // app is about to be backgrounded, as part of the Activity.onPause()
+  // flow. After this method is invoked, Chrome may be killed without further
+  // notification, so we send a pingback with data collected up to this point.
+  SendPingback(timing, info);
+  return STOP_OBSERVING;
+}
+
 void DataReductionProxyMetricsObserver::OnComplete(
     const page_load_metrics::PageLoadTiming& timing,
     const page_load_metrics::PageLoadExtraInfo& info) {