[sync] Reform struct LocalDataDescription and fix dummy data codepath

This CL does two things:
1. Renames `item_preview` field to `domains`, and introduces a new
   field `domain_count` to keep the count of the number of distinct
   domains for which there exists local data.
2. Moves the logic to return dummy data to
   SyncServiceImpl::GetLocalDataDescriptions(). This is to allow
   SyncClient implementations override the method without disturbing
   the dummy data flow.

Bug: 1451508
Change-Id: Iba751e6e5569aed9654bfcf9760f54dfe8dd2636
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4835710
Reviewed-by: Marc Treib <[email protected]>
Commit-Queue: Ankush Singh <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1192018}
diff --git a/components/sync/service/local_data_description.cc b/components/sync/service/local_data_description.cc
index 6418b04..92af727 100644
--- a/components/sync/service/local_data_description.cc
+++ b/components/sync/service/local_data_description.cc
@@ -10,9 +10,13 @@
 
 LocalDataDescription::LocalDataDescription(
     ModelType type,
-    size_t item_count,
-    const std::vector<std::string>& item_preview)
-    : type(type), item_count(item_count), item_preview(item_preview) {}
+    int item_count,
+    const std::vector<std::string>& domains,
+    int domain_count)
+    : type(type),
+      item_count(item_count),
+      domains(domains),
+      domain_count(domain_count) {}
 
 LocalDataDescription::LocalDataDescription(const LocalDataDescription&) =
     default;