Use unique_ptr to track ownership of dex files.

Bug: 18809837
Change-Id: Ie571eae8fc19ee9207390cff5c7e2a38071b126a
diff --git a/runtime/oat_file.h b/runtime/oat_file.h
index 831ba1e..6ae3c3e 100644
--- a/runtime/oat_file.h
+++ b/runtime/oat_file.h
@@ -210,7 +210,7 @@
   class OatDexFile {
    public:
     // Opens the DexFile referred to by this OatDexFile from within the containing OatFile.
-    const DexFile* OpenDexFile(std::string* error_msg) const;
+    std::unique_ptr<const DexFile> OpenDexFile(std::string* error_msg) const;
 
     const OatFile* GetOatFile() const {
       return oat_file_;