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

components/sync/test/engine/single_type_mock_server.h

Index: components/sync/test/engine/single_type_mock_server.h
diff --git a/components/sync/test/engine/single_type_mock_server.h b/components/sync/test/engine/single_type_mock_server.h
index 8c8c6fc1ced6762e48a3f744bfa8b3a78d160443..b3b4bf56a7ab094272c0fd5d06b99568a88eb252 100644
--- a/components/sync/test/engine/single_type_mock_server.h
+++ b/components/sync/test/engine/single_type_mock_server.h
@@ -74,6 +74,9 @@ class SingleTypeMockServer {
   sync_pb::DataTypeProgressMarker GetProgress() const;
   sync_pb::DataTypeContext GetContext() const;
 
+  // Sets the token that will be returned as part of GetProgress().
+  void SetProgressMarkerToken(const std::string& token);
+
  private:
   static std::string GenerateId(const std::string& tag_hash);
 
@@ -93,6 +96,9 @@ class SingleTypeMockServer {
   // Map of most recent commits by tag_hash.
   std::map<const std::string, sync_pb::SyncEntity> committed_items_;
 
+  // The token that is used to generate the current progress marker.
+  std::string progress_marker_token_;
+
   DISALLOW_COPY_AND_ASSIGN(SingleTypeMockServer);
 };