ART: Refactor callbacks_ in common_runtime_test

The callback is now important for some runtime checks, and just
resetting callbacks_ is not correct anymore.

Change-Id: I2cb806f1916fd5ea190911a833ce460be557a428
diff --git a/runtime/common_runtime_test.h b/runtime/common_runtime_test.h
index a29487f..5fbc2ee 100644
--- a/runtime/common_runtime_test.h
+++ b/runtime/common_runtime_test.h
@@ -140,10 +140,11 @@
   // Get the first dex file from a PathClassLoader. Will abort if it is null.
   const DexFile* GetFirstDexFile(jobject jclass_loader);
 
+  std::unique_ptr<CompilerCallbacks> callbacks_;
+
  private:
   static std::string GetCoreFileLocation(const char* suffix);
 
-  std::unique_ptr<CompilerCallbacks> callbacks_;
   std::vector<std::unique_ptr<const DexFile>> loaded_dex_files_;
 };