Add UKM initial_url and session_id fields.

components/ukm/ukm_source.h

Index: components/ukm/ukm_source.h
diff --git a/components/ukm/ukm_source.h b/components/ukm/ukm_source.h
index ce5ad57295f368df2acad67045382a9a28bdf190..7fa293a8ce66fb255acb42a7590dc215327fe734 100644
--- a/components/ukm/ukm_source.h
+++ b/components/ukm/ukm_source.h
@@ -25,16 +25,24 @@ class UkmSource {
   int32_t id() const { return id_; }
   void set_id(int32_t id) { id_ = id; }
 
+  const GURL& initial_url() const { return initial_url_; }
   const GURL& url() const { return url_; }
   void set_url(const GURL& url) { url_ = url; }
+  void UpdateUrl(const GURL& url);

[rkaplow, 2017/03/02 18:46:54]

can you comment on this (with some init url details)

[Bryan McQuade, 2017/03/02 22:18:32]

Done

 
   // Serializes the members of the class into the supplied proto.
   void PopulateProto(Source* proto_source) const;
 
  private:
   int32_t id_;
+
+  // The final, canonical URL for this source.
   GURL url_;
 
+  // The initial URL for this source. Only set if different from |url_| (i.e. if
+  // the URL changed over the lifetime of this source).
+  GURL initial_url_;
+
   DISALLOW_COPY_AND_ASSIGN(UkmSource);
 };