[NTP::SectionOrder] Replace CategoryFactory with a category ranker.

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 0ae86c8b16f9e7d2fc5f00d62166dd9c883e34ec..a9b74827ae9758dec74ef20f5b843f445199cff9 100644
--- a/components/ntp_snippets/category.h
+++ b/components/ntp_snippets/category.h
@@ -9,8 +9,6 @@
 
 namespace ntp_snippets {
 
-class CategoryFactory;
-
 // These are the categories that the client knows about.
 // The values before LOCAL_CATEGORIES_COUNT are the categories that are provided
 // locally on the device. Categories provided by the server (IDs strictly larger
@@ -67,9 +65,20 @@ class Category {
   // Returns whether this category matches the given |known_category|.
   bool IsKnownCategory(KnownCategories known_category) const;
 
- private:
-  friend class CategoryFactory;
+  // Creates a category from a KnownCategory value. The passed |known_category|
+  // must not be one of the special values (LOCAL_CATEGORIES_COUNT or
+  // REMOTE_CATEGORIES_OFFSET).
+  static Category FromKnownCategory(KnownCategories known_category);
+
+  // Creates a category from a category identifier delivered by the server.
+  // |remote_category| must be positive.
+  static Category FromRemoteCategory(int remote_category);
 
+  // Creates a category from an ID as returned by |Category::id()|.

[Marc Treib, 2016/12/13 12:22:42]

nit: "Category::" isn't required

[vitaliii, 2016/12/14 08:59:38]

Done.

+  // |id| must be a non-negative value.
+  static Category FromIDValue(int id);

[Marc Treib, 2016/12/13 12:22:42]

nit: Move all the static "create" methods to the top?

[vitaliii, 2016/12/14 08:59:38]

Done.

+
+ private:
   explicit Category(int id);
 
   int id_;