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

chrome/browser/page_load_metrics/observers/data_reduction_proxy_metrics_observer.h

 // 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.
 
 #ifndef CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_DATA_REDUCTION_PROXY_METRICS_OBSERVER_H_
 #define CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_DATA_REDUCTION_PROXY_METRICS_OBSERVER_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 41 matching lines @@
   ~DataReductionProxyMetricsObserver() override;
 
   // page_load_metrics::PageLoadMetricsObserver:
   ObservePolicy OnStart(content::NavigationHandle* navigation_handle,
                         const GURL& currently_committed_url,
                         bool started_in_foreground) override;
   ObservePolicy OnCommit(content::NavigationHandle* navigation_handle) override;
   ObservePolicy OnHidden(
       const page_load_metrics::PageLoadTiming& timing,
       const page_load_metrics::PageLoadExtraInfo& info) override;
+  ObservePolicy FlushMetricsOnAppEnterBackground(
+      const page_load_metrics::PageLoadTiming& timing,
+      const page_load_metrics::PageLoadExtraInfo& info) override;
   void OnComplete(const page_load_metrics::PageLoadTiming& timing,
                   const page_load_metrics::PageLoadExtraInfo& info) override;
   void OnDomContentLoadedEventStart(
       const page_load_metrics::PageLoadTiming& timing,
       const page_load_metrics::PageLoadExtraInfo& info) override;
   void OnLoadEventStart(
       const page_load_metrics::PageLoadTiming& timing,
       const page_load_metrics::PageLoadExtraInfo& info) override;
   void OnFirstLayout(const page_load_metrics::PageLoadTiming& timing,
                      const page_load_metrics::PageLoadExtraInfo& info) override;
@@ skipping 29 matching lines @@
 
   // The browser context this navigation is operating in.
   content::BrowserContext* browser_context_;
 
   DISALLOW_COPY_AND_ASSIGN(DataReductionProxyMetricsObserver);
 };
 
 }  // namespace data_reduction_proxy
 
 #endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_DATA_REDUCTION_PROXY_METRICS_OBSERVER_H_