[Sync] Adding integration tests for USS encryption and fixing a worker bug.

components/sync/test/engine/single_type_mock_server.cc

Index: components/sync/test/engine/single_type_mock_server.cc
diff --git a/components/sync/test/engine/single_type_mock_server.cc b/components/sync/test/engine/single_type_mock_server.cc
index e3d3dc9c3bac9b34a663410f60accfd9a69d19ce..efddfd0d0781b61337d2492b7b720517bfbf4108 100644
--- a/components/sync/test/engine/single_type_mock_server.cc
+++ b/components/sync/test/engine/single_type_mock_server.cc
@@ -11,7 +11,9 @@ using google::protobuf::RepeatedPtrField;
 namespace syncer {
 
 SingleTypeMockServer::SingleTypeMockServer(ModelType type)
-    : type_(type), type_root_id_(ModelTypeToRootTag(type)) {}
+    : type_(type),
+      type_root_id_(ModelTypeToRootTag(type)),
+      progress_marker_token_("non_null_progress_token") {}
 
 SingleTypeMockServer::~SingleTypeMockServer() {}
 
@@ -144,7 +146,7 @@ sync_pb::SyncEntity SingleTypeMockServer::GetLastCommittedEntity(
 sync_pb::DataTypeProgressMarker SingleTypeMockServer::GetProgress() const {
   sync_pb::DataTypeProgressMarker progress;
   progress.set_data_type_id(type_);
-  progress.set_token("non_null_progress_token");
+  progress.set_token(progress_marker_token_);
   return progress;
 }
 
@@ -152,6 +154,10 @@ sync_pb::DataTypeContext SingleTypeMockServer::GetContext() const {
   return sync_pb::DataTypeContext();
 }
 
+void SingleTypeMockServer::SetProgressMarkerToken(const std::string& token) {
+  progress_marker_token_ = token;
+}
+
 std::string SingleTypeMockServer::GenerateId(const std::string& tag_hash) {
   return "FakeId:" + tag_hash;
 }