UKM: Added support for navigation sources

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 44afe3a737b1905068e331a3a2f2c97045e8f323..e48f252cc9380fb6080affe4baa73ae9f9fc5470 100644
--- a/components/metrics/proto/ukm/source.proto
+++ b/components/metrics/proto/ukm/source.proto
@@ -24,6 +24,6 @@ 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 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;
 }