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

chrome/browser/page_load_metrics/metrics_web_contents_observer.cc

 // Copyright 2015 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/metrics_web_contents_observer.h"
 
 #include <algorithm>
 #include <ostream>
 #include <string>
 #include <utility>
@@ skipping 290 matching lines @@
     const blink::WebInputEvent& event) {
   // Ignore browser navigation or reload which comes with type Undefined.
   if (event.type == blink::WebInputEvent::Type::Undefined)
     return;
 
   if (committed_load_)
     committed_load_->OnInputEvent(event);
 }
 
 void MetricsWebContentsObserver::FlushMetricsOnAppEnterBackground() {
-  // Signal to observers that we've been backgrounded, in cases where the
-  // FlushMetricsOnAppEnterBackground callback gets invoked before the
-  // associated WasHidden callback.
-  WasHidden();
+  // Note that, while a call to FlushMetricsOnAppEnterBackground usually
+  // indicates that the app is about to be backgrounded, there are cases where
+  // the app may not end up getting backgrounded. Thus, we should not assume
+  // anything about foreground / background state of the associated tab as part
+  // of this method call.
 
   if (committed_load_)
     committed_load_->FlushMetricsOnAppEnterBackground();
   for (const auto& kv : provisional_loads_) {
     kv.second->FlushMetricsOnAppEnterBackground();
   }
   for (const auto& tracker : aborted_provisional_loads_) {
     tracker->FlushMetricsOnAppEnterBackground();
   }
 }
@@ skipping 148 matching lines @@
     content::NavigationHandle* navigation_handle) const {
   DCHECK(navigation_handle->IsInMainFrame());
   DCHECK(!navigation_handle->HasCommitted() ||
          !navigation_handle->IsSamePage());
 
   return BrowserPageTrackDecider(embedder_interface_.get(), web_contents(),
                                  navigation_handle).ShouldTrack();
 }
 
 }  // namespace page_load_metrics