Modernize some extensions code.

extensions/browser/extension_event_histogram_value.h

Index: extensions/browser/extension_event_histogram_value.h
diff --git a/extensions/browser/extension_event_histogram_value.h b/extensions/browser/extension_event_histogram_value.h
index 0c9bafebc92641602c05e54a6e0368ae231540d1..c761918d06ebed60018cd3cca0429d3614ec935f 100644
--- a/extensions/browser/extension_event_histogram_value.h
+++ b/extensions/browser/extension_event_histogram_value.h
@@ -8,9 +8,6 @@
 namespace extensions {
 namespace events {
 
-// TODO(kalman): I am still in the process of migrating Event construction away
-// from using "UNKNOWN" to their real histogram values. See crbug.com/503402.
-//
 // Short version:
 //  *Never* reorder or delete entries in the |HistogramValue| enumeration.
 //  When creating a new extension event, add a new entry at the end of the