TabRestore page load metrics does not track app background UMA properly

chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.h

Index: chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.h
diff --git a/chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.h b/chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.h
index 4df6470fce4823aedbd34e4fde8a5f0b082aae06..67d244ef870853997f94d2cbaf29e7f6d2366c78 100644
--- a/chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.h
+++ b/chrome/browser/page_load_metrics/observers/page_load_metrics_observer_test_harness.h
@@ -62,6 +62,9 @@ class PageLoadMetricsObserverTestHarness
   // Simulates a user input.
   void SimulateInputEvent(const blink::WebInputEvent& event);
 
+  // Simulates the app being backgrounded.
+  void SimulateAppEnterBackground();
+
   const base::HistogramTester& histogram_tester() const;
 
   // Gets the PageLoadExtraInfo for the committed_load_ in observer_.