[Suggestions category] Warn more about removing 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 6e36a3aa47ec5c73672e28826bcc305ea8ee7e82..a8571cfaa86852fed8bfaaa009dc99de83a3104f 100644
--- a/components/ntp_snippets/category.h
+++ b/components/ntp_snippets/category.h
@@ -33,7 +33,12 @@ enum class KnownCategories {
 
   // Pages recently browsed to on other devices.
   FOREIGN_TABS,
-  // INSERT NEW LOCAL CATEGORIES HERE!
+
+  // ****************** INSERT NEW LOCAL CATEGORIES HERE! ******************
+  // Existing categories must never be removed. This happens easily if you
+  // switch out of a local branch that contains a new category. If you cannot
+  // avoid it, consider forcing ConstantCategoryRanker via chrome://flags.
+  // Otherwise, Chrome will crash and you would need to reinstall it to fix it.

[vitaliii, 2017/02/08 08:40:35]

Jumping directly to "switching out of a local branch" seems premature. This is
just one example of "removing a category implicitly", I would like to point this
out explicitly instead of an example. Also "switching out of a branch" does not
cause this by itself, you also need to install the app, so this is very indirect
way of saying what is going on.
I do not like giving advice on how to avoid this. There may be other places
where this information is persisted, so this comment is likely to become
outdated (and misleading). Also this is not Android-only code, so I am not sure
whether "Chrome will crash" is always true.

Overall, I would go for:

" Existing categories are persisted and they must never be removed. This may
happen implicitly, e.g. when an older version without some local category is
installed. "

As for development, you can make a parent branch with your new category and make
all other branches depend on it. Or you probably could temporarily add your
category as remote.

[jkrcal, 2017/02/08 10:12:05]

Done.

 
   // Follows the last local category.
   LOCAL_CATEGORIES_COUNT,
@@ -43,7 +48,8 @@ enum class KnownCategories {
 
   // Articles for you.
   ARTICLES,
-  // INSERT NEW REMOTE CATEGORIES HERE!
+
+  // ****************** INSERT NEW REMOTE CATEGORIES HERE! ******************
 
   // Tracks the last known remote category
   LAST_KNOWN_REMOTE_CATEGORY = ARTICLES,