XXS: Update a comment in the safebrowsing.proto file

The comment states that although the field is marked as repeated, in practice it
can only contain 0 or 1 values.

BUG=543161

Review-Url: https://codereview.chromium.org/2195043002
Cr-Commit-Position: refs/heads/master@{#408767}

components/safe_browsing_db/safebrowsing.proto

@@ -149,8 +149,8 @@ message FetchThreatListUpdatesResponse {
     // response.
     repeated ThreatEntrySet additions = 5;
 
-    // A set of entries to remove from a local threat type's list. Repeated for
-    // the same reason as above.
+    // A set of entries to remove from a local threat type's list. In practice,
+    // this field is empty or contains exactly one ThreatEntrySet.
     repeated ThreatEntrySet removals = 6;
 
     // The new client state, in encrypted format. Opaque to clients.