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..6834746810143c3e846fd35ef11a9ac2752b8b41 100644
--- a/components/ukm/ukm_source.h
+++ b/components/ukm/ukm_source.h
@@ -25,16 +25,32 @@ 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_; }
+
+  // Sets the URL for this source. Should be invoked when a source is
+  // initialized.
   void set_url(const GURL& url) { url_ = url; }
 
+  // Updates the URL for this source. Must be called after set_url. If a new URL
+  // is passed to UpdateUrl, the initial_url field is populated with the
+  // original URL provided to set_url, and the url field is updated with the
+  // value provided to this method.
+  void UpdateUrl(const GURL& url);
+
   // 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);
 };