[NTP Snippets] Metrics: switch over known categories

components/ntp_snippets/category.h

Index: components/ntp_snippets/category.h
diff --git a/components/ntp_snippets/category.h b/components/ntp_snippets/category.h
index 524986fb3b8e5af8b93a02bc87cf341d80d80a00..fafcde1e69adc0c49ef470a24e278d2ae1e1ba3a 100644
--- a/components/ntp_snippets/category.h
+++ b/components/ntp_snippets/category.h
@@ -34,6 +34,7 @@ enum class KnownCategories {
   // Follows the last local category.
   LOCAL_CATEGORIES_COUNT,
 
+  // Remote categories come after this.
   REMOTE_CATEGORIES_OFFSET = 10000,
 
   // Articles for you.
@@ -55,6 +56,7 @@ class Category {
   // the application, so they should not be persisted.
   int id() const { return id_; }
 
+  // Returns whether this category matches the given |known_category|.
   bool IsKnownCategory(KnownCategories known_category) const;
 
  private: