Add UKM initial_url and session_id fields.

components/metrics/proto/ukm/source.proto

Index: components/metrics/proto/ukm/source.proto
diff --git a/components/metrics/proto/ukm/source.proto b/components/metrics/proto/ukm/source.proto
index 2f6dc1f6afe41ec1c0464299539232568814ec9d..2fe543e789cf7de52d3bdf6a0da88adb40561ac9 100644
--- a/components/metrics/proto/ukm/source.proto
+++ b/components/metrics/proto/ukm/source.proto
@@ -9,7 +9,7 @@ package ukm;
 option optimize_for = LITE_RUNTIME;
 
 // Source contains data related to a top-level navigation.
-// Next tag: 6
+// Next tag: 7
 message Source {
   // An identifier for the source. This should be unique within a session.
   optional int32 id = 1;
@@ -22,6 +22,11 @@ message Source {
   reserved 5;
   reserved "url_origin";
 
+  // The initial URL of the source. Set in cases where the source URL changed
+  // (e.g. it was redirected), otherwise unset. If this URL has not been
+  // discovered by Google's crawler, it should not be recorded.
+  optional string initial_url = 6;
+
   // Timestamp of navigation to this Source, as seen by the client. Time of
   // events related to this Source will generally be relative to this timestamp.
   optional int64 navigation_time_msec = 3;