Use unique_ptr to track ownership of dex files.

Bug: 18809837
Change-Id: Ie571eae8fc19ee9207390cff5c7e2a38071b126a
diff --git a/runtime/oat_file.cc b/runtime/oat_file.cc
index 1c6cc8b..358519b 100644
--- a/runtime/oat_file.cc
+++ b/runtime/oat_file.cc
@@ -454,7 +454,7 @@
   return reinterpret_cast<const DexFile::Header*>(dex_file_pointer_)->file_size_;
 }
 
-const DexFile* OatFile::OatDexFile::OpenDexFile(std::string* error_msg) const {
+std::unique_ptr<const DexFile> OatFile::OatDexFile::OpenDexFile(std::string* error_msg) const {
   return DexFile::Open(dex_file_pointer_, FileSize(), dex_file_location_,
                        dex_file_location_checksum_, error_msg);
 }