[core] Fix performance-unnecessary-value-param errors in header files

As reported by clang-tidy-8.

1 files changed, 1 insertions, 1 deletions

diff --git a/test/renderer/image_manager.test.cpp b/test/renderer/image_manager.test.cpp
index c3ab0d8227..395c9fe1e3 100644
--- a/test/renderer/image_manager.test.cpp
+++ b/test/renderer/image_manager.test.cpp
@@ -140,7 +140,7 @@ class StubImageManagerObserver : public ImageManagerObserver {
     public:
     int count = 0;
     std::function<void (const std::string&)> imageMissing = [](const std::string&){};
-    void onStyleImageMissing(const std::string& id, std::function<void()> done) override {
+    void onStyleImageMissing(const std::string& id, const std::function<void()>& done) override {
         count++;
         imageMissing(id);
         done();