Use unique_ptr to track ownership of dex files.

Bug: 18809837
Change-Id: Ie571eae8fc19ee9207390cff5c7e2a38071b126a
diff --git a/runtime/runtime.h b/runtime/runtime.h
index e319963..d58fe3c 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -425,6 +425,9 @@
       LOCKS_EXCLUDED(method_verifier_lock_);
 
   const std::vector<const DexFile*>& GetCompileTimeClassPath(jobject class_loader);
+
+  // The caller is responsible for ensuring the class_path DexFiles remain
+  // valid as long as the Runtime object remains valid.
   void SetCompileTimeClassPath(jobject class_loader, std::vector<const DexFile*>& class_path);
 
   void StartProfiler(const char* profile_output_filename);