[USS] implement CreateMetadataChangeList and GetClientTag

components/history/core/browser/history_backend.h

Index: components/history/core/browser/history_backend.h
diff --git a/components/history/core/browser/history_backend.h b/components/history/core/browser/history_backend.h
index c10b1e0d3814008972d21cada5b056424c6037bf..e99a0b7d442e85e2759759a56dd391f74991b8a8 100644
--- a/components/history/core/browser/history_backend.h
+++ b/components/history/core/browser/history_backend.h
@@ -33,6 +33,7 @@
 #include "components/history/core/browser/keyword_id.h"
 #include "components/history/core/browser/thumbnail_database.h"
 #include "components/history/core/browser/visit_tracker.h"
+#include "components/sync/model/sync_metadata_store.h"
 #include "sql/init_status.h"
 
 class SkBitmap;
@@ -99,7 +100,8 @@ class QueuedHistoryDBTask {
 // functions in the history service. These functions are not documented
 // here, see the history service for behavior.
 class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
-                       public HistoryBackendNotifier {
+                       public HistoryBackendNotifier,
+                       public syncer::SyncMetadataStore {

[pavely, 2017/05/01 17:24:16]

I don't really like exposing SyncMetadataStore to consumers of HistoryBackend.
What if instead we passed pointer to SyncMetadataStore into constructor of
TypedURLSyncBridge? We instantiate bridge in HistoryBackend::Init where we can
get the reference to HistoryDatabase.

  public:
   // Interface implemented by the owner of the HistoryBackend object. Normally,
   // the history service implements this to send stuff back to the main thread.
@@ -484,6 +486,17 @@ class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
 
   base::Time GetFirstRecordedTimeForTest() { return first_recorded_time_; }
 
+  // syncer::SyncMetadataStore implementation.
+  bool UpdateSyncMetadata(syncer::ModelType model_type,
+                          const std::string& storage_key,
+                          const sync_pb::EntityMetadata& metadata) override;
+  bool ClearSyncMetadata(syncer::ModelType model_type,
+                         const std::string& storage_key) override;
+  bool UpdateModelTypeState(
+      syncer::ModelType model_type,
+      const sync_pb::ModelTypeState& model_type_state) override;
+  bool ClearModelTypeState(syncer::ModelType model_type) override;
+
  protected:
   ~HistoryBackend() override;