[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..49b50d0e15791ee9921e520fb819c9e93f25ff42 100644
--- a/components/ntp_snippets/category.h
+++ b/components/ntp_snippets/category.h
@@ -16,6 +16,7 @@ class CategoryFactory;
 // locally on the device. Categories provided by the server (IDs strictly larger
 // than REMOTE_CATEGORIES_OFFSET) only need to be hard-coded here if they need
 // to be recognized by the client implementation.
+// NOTE: If you add an entry here, also add it to |IsAnyKnownCategory|.
 // On Android builds, a Java counterpart will be generated for this enum.
 // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.chrome.browser.ntp.snippets
 enum class KnownCategories {
@@ -55,8 +56,12 @@ 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;
 
+  // Returns whether this category is any of the known categories.

[jkrcal, 2016/09/13 16:08:05]

Without looking in the implementation, I would not understand what you mean by
"is any known category". Maybe expand by ", i.e. any element of the
KnowCategories enum except for LOCAL_CATEGORIES_COUNT and
REMOTE_CATEGORIES_OFFSET." 

It is also inconsistent with IsKnownCategory which accepts these two cases as
known categories. Admittedly, only an idiot would ask if this Category is the
LOCAL_CATEGORIES_COUNT-category. Still, I would prefer IsKnownCategory to
blacklist these two cases to get the code consistent and self-documented.

[Marc Treib, 2016/09/13 16:28:50]

Added, except for the "except..." part. It should be impossible to create such a
Category instance (see CategoryFactory::FromKnownCategory), so IMO that part of
the comment isn't necessary.
Again, it shouldn't be possible to create those categories. Still, I've added
DCHECKs in IsKnownCategory to make clear that those aren't valid values.

[jkrcal, 2016/09/13 17:22:28]

Agreed, thanks.

+  bool IsAnyKnownCategory() const;
+
  private:
   friend class CategoryFactory;