UKM: Added support for navigation sources

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: 5
 message Source {
   // A identifier for the source. This should be unique even across sessions.
   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 must be stripped to the security origin
   // before recording to logs.  If the security origin has not been discovered
   // by the crawler, the security origin must not be logged.
   optional string url = 2;
 
   // 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 meaningful paint, in milliseconds.
-  optional int64 first_meaningful_paint_msec = 4;
+  // Time between navigation and the first contentful paint, in milliseconds.
+  optional int64 first_contentful_paint_msec = 4;
 }