Convert first contentful paint logging to the new UKM client API

components/metrics/proto/ukm/source.proto

Index: components/metrics/proto/ukm/source.proto
diff --git a/components/metrics/proto/ukm/source.proto b/components/metrics/proto/ukm/source.proto
index 165083bf6d6aafc7081f93df19d71fea6b68e34d..2f6dc1f6afe41ec1c0464299539232568814ec9d 100644
--- a/components/metrics/proto/ukm/source.proto
+++ b/components/metrics/proto/ukm/source.proto
@@ -26,6 +26,7 @@ message Source {
   // 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];
 }