Use multiple IDs when discarding or merging snippets.

components/ntp_snippets/ntp_snippet.h

Index: components/ntp_snippets/ntp_snippet.h
diff --git a/components/ntp_snippets/ntp_snippet.h b/components/ntp_snippets/ntp_snippet.h
index 20c7fe732711a6b4d48d433529d4eff0c58a62d7..d741d7fa8b202edbca39bea3435332e52b285961 100644
--- a/components/ntp_snippets/ntp_snippet.h
+++ b/components/ntp_snippets/ntp_snippet.h
@@ -57,6 +57,9 @@ class NTPSnippet {
   // CreateFromDictionary() the relevant key is 'url'.
   // TODO(treib): For now, the ID has to be a valid URL spec, otherwise
   // fetching the salient image will fail. See TODO in ntp_snippets_service.cc.
+  // TODO(tschumann): With the new data layout based on the zine backend service
+  // protos, we should distinguish between an ID (should only be used by the JNI
+  // bridge) and a URL (used by snippets internal page etc.).
   const std::string& id() const { return id_; }
 
   // Title of the snippet.