[Sync] MergeSyncData implementation for Autocomplete.

components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc

Index: components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc
diff --git a/components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc b/components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc
index 0bf47d314ba2ab9ea4a087443d69be3858b8fc3f..c5ce09b1a3cfff9dc4b994f15de04f3bd11c316a 100644
--- a/components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc
+++ b/components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc
@@ -143,17 +143,19 @@ class SyncDifferenceTracker {
       return ModelError(FROM_HERE, "Failed reading from WebDatabase.");
     } else if (!local) {
       save_to_local_.push_back(remote);
-    } else if (remote != local.value()) {
-      if (specifics.usage_timestamp().empty()) {
-        // Skip merging if there are no timestamps. We don't want to wipe out
-        // a local value of |date_created| if the remote copy is oddly formed.
-        save_to_sync_.push_back(local.value());
-      } else {
-        const AutofillEntry merged = MergeEntryDates(local.value(), remote);
-        save_to_local_.push_back(merged);
-        save_to_sync_.push_back(merged);
-      }
+    } else {
       unique_to_local_.erase(local.value());
+      if (remote != local.value()) {
+        if (specifics.usage_timestamp().empty()) {
+          // Skip merging if there are no timestamps. We don't want to wipe out
+          // a local value of |date_created| if the remote copy is oddly formed.
+          save_to_sync_.push_back(local.value());
+        } else {
+          const AutofillEntry merged = MergeEntryDates(local.value(), remote);
+          save_to_local_.push_back(merged);
+          save_to_sync_.push_back(merged);
+        }
+      }
     }
     return {};
   }
@@ -318,8 +320,7 @@ Optional<syncer::ModelError> AutocompleteSyncBridge::MergeSyncData(
     EntityDataMap entity_data_map) {
   DCHECK(thread_checker_.CalledOnValidThread());
 
-  // TODO(skym, crbug.com/680218): Uncomment and add unit tests.
-  /*SyncDifferenceTracker tracker(GetAutofillTable());
+  SyncDifferenceTracker tracker(GetAutofillTable());
   for (auto kv : entity_data_map) {
     DCHECK(kv.second->specifics.has_autofill());
     RETURN_IF_ERROR(tracker.IncorporateRemoteSpecifics(
@@ -330,7 +331,7 @@ Optional<syncer::ModelError> AutocompleteSyncBridge::MergeSyncData(
   RETURN_IF_ERROR(tracker.FlushToSync(true, std::move(metadata_change_list),
                                 change_processor()));
   web_data_backend_->RemoveExpiredFormElements();
-  web_data_backend_->NotifyThatSyncHasStarted(syncer::AUTOFILL);*/
+  web_data_backend_->NotifyThatSyncHasStarted(syncer::AUTOFILL);
   return {};
 }