Add support for counting same-document AMP loads.

chrome/browser/page_load_metrics/page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/page_load_metrics_observer.cc
index 181c30de2d376a2d3e5ea9fcb644900d2fd0d1f9..2a51ccbdcf57f8ef79caa3ad2cc8f30c251e85d2 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/page_load_metrics_observer.cc
@@ -106,12 +106,6 @@ PageLoadMetricsObserver::ObservePolicy PageLoadMetricsObserver::OnCommit(
   return CONTINUE_OBSERVING;
 }
 
-PageLoadMetricsObserver::ObservePolicy
-PageLoadMetricsObserver::OnDidFinishSubFrameNavigation(
-    content::NavigationHandle* navigation_handle) {
-  return CONTINUE_OBSERVING;
-}
-
 PageLoadMetricsObserver::ObservePolicy PageLoadMetricsObserver::OnHidden(
     const PageLoadTiming& timing,
     const PageLoadExtraInfo& extra_info) {