Convert first contentful paint logging to the new UKM client API

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 7acf510869d9f70fc928dd4e7859519e206ec3e3..67aa6fb6a7c7eab995971c32a9335c44b7f2f3d3 100644
--- a/components/ukm/ukm_source.h
+++ b/components/ukm/ukm_source.h
@@ -26,20 +26,11 @@ class UkmSource {
   const content::GlobalRequestID& id() const { return id_; }
   void set_id(const content::GlobalRequestID& id) { id_ = id; }
 
-  const GURL& committed_url() const { return committed_url_; }
-  void set_committed_url(const GURL& committed_url) {
-    committed_url_ = committed_url;
-  }
-
-  base::TimeDelta first_contentful_paint() const {
-    return first_contentful_paint_;
-  }
-  void set_first_contentful_paint(base::TimeDelta first_contentful_paint) {
-    first_contentful_paint_ = first_contentful_paint;
-  }
+  const GURL& url() const { return url_; }
+  void set_url(const GURL& url) { url_ = url; }
 
   // Serializes the members of the class into the supplied proto.
-  void PopulateProto(Source* proto_source);
+  void PopulateProto(Source* proto_source) const;
 
   // Get the hashed source ID, which is the hash of the child and request id
   // pairs from content::GlobalRequestID.
@@ -47,8 +38,7 @@ class UkmSource {
 
  private:
   content::GlobalRequestID id_;
-  GURL committed_url_;
-  base::TimeDelta first_contentful_paint_;
+  GURL url_;
 
   DISALLOW_COPY_AND_ASSIGN(UkmSource);
 };