Plumb experiment flags through page_load_metrics and add a new observer

components/page_load_metrics/browser/page_load_metrics_observer.h

Index: components/page_load_metrics/browser/page_load_metrics_observer.h
diff --git a/components/page_load_metrics/browser/page_load_metrics_observer.h b/components/page_load_metrics/browser/page_load_metrics_observer.h
index c42f36d93ee467eab1375a7bad8abbb8b90a4441..659eaa59b3bcfc3ac24489b0effe1f0b272e39cd 100644
--- a/components/page_load_metrics/browser/page_load_metrics_observer.h
+++ b/components/page_load_metrics/browser/page_load_metrics_observer.h
@@ -57,7 +57,8 @@ struct PageLoadExtraInfo {
                     const GURL& committed_url,
                     base::TimeDelta time_to_commit,
                     UserAbortType abort_type,
-                    base::TimeDelta time_to_abort);
+                    base::TimeDelta time_to_abort,
+                    const PageLoadMetadata& metadata);
 
   // The first time that the page was backgrounded since the navigation started.
   // If the page has not been backgrounded this will be base::TimeDelta().
@@ -83,6 +84,10 @@ struct PageLoadExtraInfo {
   // |base::TimeDelta()|.
   const UserAbortType abort_type;
   const base::TimeDelta time_to_abort;
+
+  // Extra information supplied to the page load metrics system from the
+  // renderer.
+  const PageLoadMetadata metadata;
 };
 
 // Interface for PageLoadMetrics observers. All instances of this class are