[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..da2db885f7b3377630fa27c397f71d37b1f2e706 100644
--- a/sync/test/engine/mock_update_handler.cc
+++ b/sync/test/engine/mock_update_handler.cc
@@ -24,10 +24,16 @@ 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,
-      sessions::StatusController* status) {
+    const sync_pb::DataTypeProgressMarker& progress_marker,
+    const sync_pb::DataTypeContext& mutated_context,
+    const SyncEntityList& applicable_updates,
+    sessions::StatusController* status) {
   progress_marker_.CopyFrom(progress_marker);
 }