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

chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.h"
 
 #include <string>
 
 #include "base/optional.h"
 #include "base/time/time.h"
@@ skipping 114 matching lines @@
 }
 
 page_load_metrics::PageLoadMetricsObserver::ObservePolicy
 DataReductionProxyMetricsObserver::OnHidden(
     const page_load_metrics::PageLoadTiming& timing,
     const page_load_metrics::PageLoadExtraInfo& info) {
   SendPingback(timing, info);
   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) {
   SendPingback(timing, info);
 }
 
 void DataReductionProxyMetricsObserver::SendPingback(
     const page_load_metrics::PageLoadTiming& timing,
     const page_load_metrics::PageLoadExtraInfo& info) {
   // TODO(ryansturm): Move to OnFirstBackgroundEvent to handle some fast
@@ skipping 129 matching lines @@
 
 DataReductionProxyPingbackClient*
 DataReductionProxyMetricsObserver::GetPingbackClient() const {
   return DataReductionProxyChromeSettingsFactory::GetForBrowserContext(
              browser_context_)
       ->data_reduction_proxy_service()
       ->pingback_client();
 }
 
 }  // namespace data_reduction_proxy