[Sync] Add plumbing of context from client to server

sync/test/engine/mock_update_handler.cc

Index: sync/test/engine/mock_update_handler.cc
diff --git a/sync/test/engine/mock_update_handler.cc b/sync/test/engine/mock_update_handler.cc
index 098086e60ae427b08c3e9b7d0e18cd30b6587069..af39ddfae3ca9bb60bd68c1b292fb427f93a1783 100644
--- a/sync/test/engine/mock_update_handler.cc
+++ b/sync/test/engine/mock_update_handler.cc
@@ -24,6 +24,11 @@ void MockUpdateHandler::GetDownloadProgress(
   progress_marker->CopyFrom(progress_marker_);
 }
 
+void MockUpdateHandler::GetDataTypeContext(
+    sync_pb::DataTypeContext* context) const {
+  context->Clear();
+}
+
 void MockUpdateHandler::ProcessGetUpdatesResponse(
       const sync_pb::DataTypeProgressMarker& progress_marker,
       const SyncEntityList& applicable_updates,
@@ -31,6 +36,10 @@ void MockUpdateHandler::ProcessGetUpdatesResponse(
   progress_marker_.CopyFrom(progress_marker);
 }
 
+void MockUpdateHandler::ProcessDataTypeContextMutation(
+      const sync_pb::DataTypeContext& mutated_context) {
+}
+
 void MockUpdateHandler::ApplyUpdates(sessions::StatusController* status) {
   apply_updates_count_++;
 }