Convert first contentful paint logging to the new UKM client API

components/metrics/proto/ukm/source.proto

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 package ukm;
 
 option optimize_for = LITE_RUNTIME;
 
 // Source contains data related to a top-level navigation.
 // Next tag: 6
 message Source {
   // An identifier for the source. This should be unique within a session.
   optional int32 id = 1;
 
   // The URL of the source, as recorded in history. If this URL has not been
   // discovered by Google's crawler, it should not be recorded.
   optional string url = 2;
 
   // The origin of the URL. This is reserved to be used by server side only.
   reserved 5;
   reserved "url_origin";
 
   // 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;
 
-  // Time between navigation and the first contentful paint, in milliseconds.
-  optional int64 first_contentful_paint_msec = 4;
+  // Time between navigation and the first contentful paint, in
+  // milliseconds. Deprecated. Use ukm::Entry instead.
+  optional int64 first_contentful_paint_msec = 4 [deprecated = true];
 }