Add UKM initial_url and session_id fields.

components/ukm/ukm_service.h

Index: components/ukm/ukm_service.h
diff --git a/components/ukm/ukm_service.h b/components/ukm/ukm_service.h
index 540d4c5eefd64fa0fbb9671df66d80f620d653a3..3431b672bbc360c1fc6ea357e246cf4166ffb53c 100644
--- a/components/ukm/ukm_service.h
+++ b/components/ukm/ukm_service.h
@@ -158,6 +158,9 @@ class UkmService : public base::SupportsWeakPtr<UkmService> {
   // The UKM client id stored in prefs.
   uint64_t client_id_;
 
+  // The UKM session id stored in prefs.
+  int32_t session_id_;
+
   // Used to interact with the embedder. Weak pointer; must outlive |this|
   // instance.
   metrics::MetricsServiceClient* const client_;