[Suggestions category] Warn more about removing categories.

components/ntp_snippets/category.cc

Index: components/ntp_snippets/category.cc
diff --git a/components/ntp_snippets/category.cc b/components/ntp_snippets/category.cc
index 7274a0105c4854aa0bb69320e9b0b2d13812a53b..1f7a77d9f4545a577a4cc60b1aa99baa25500245 100644
--- a/components/ntp_snippets/category.cc
+++ b/components/ntp_snippets/category.cc
@@ -22,7 +22,10 @@ Category Category::FromRemoteCategory(int remote_category) {
 
 // static
 Category Category::FromIDValue(int id) {
-  DCHECK(IsValidIDValue(id)) << "Not a valid ID: " << id;
+  DCHECK(IsValidIDValue(id))
+      << "Not a valid ID: " << id
+      << " Note that local categories must not be removed. This can happen if "
+         "you switch out of a branch with a new local category.";

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

I have multiple concerns regarding this message:
1) It does not look like a sentence:
"Not a valid ID: 13 Note that local categories..."

2) Too much attention to category removal. There may be other reasons for this
DCHECK to fail, also shuffling categories is bad as well.

3) The last sentence seems very difficult to understand. I think it is enough to
point the reader into right direction (KnownCategories) and have more elaborate
message there.

Taking all of these into account, I would go for:

"id << is not a valid category ID. This may have been caused by removal of a
local KnownCategory."

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

Done.

   return Category(id);
 }