Added UkmPageLoadMetricsObserver which sends top-level metrics to UKM

components/ukm/ukm_source.cc

Index: components/ukm/ukm_source.cc
diff --git a/components/ukm/ukm_source.cc b/components/ukm/ukm_source.cc
index ca0fc0a49da8b1a847cd9b400e05e88d1432bebb..dc2f9e033a4f84435e34b0adf0892b661c9a01d9 100644
--- a/components/ukm/ukm_source.cc
+++ b/components/ukm/ukm_source.cc
@@ -14,11 +14,6 @@ UkmSource::~UkmSource() = default;
 void UkmSource::PopulateProto(Source* proto_source) {
   proto_source->set_url(committed_url_.spec());
 
-  base::TimeDelta navigation_start_in_msec =
-      navigation_start_ - base::Time::UnixEpoch();
-  proto_source->set_navigation_time_msec(
-      navigation_start_in_msec.InMilliseconds());
-
   proto_source->set_first_contentful_paint_msec(
       first_contentful_paint_.InMilliseconds());
 }