Fix class_linker_test for valgrind.

Fix a memory leak.

Bug: 27156726
Change-Id: I4d9a2dc9cff447b6f2fd02432e018ae49766563a
diff --git a/runtime/class_linker_test.cc b/runtime/class_linker_test.cc
index 4d528ed..5c3029a 100644
--- a/runtime/class_linker_test.cc
+++ b/runtime/class_linker_test.cc
@@ -1225,12 +1225,12 @@
   dex_cache->SetLocation(location.Get());
   const DexFile* old_dex_file = dex_cache->GetDexFile();
 
-  DexFile* dex_file = new DexFile(old_dex_file->Begin(),
-                                  old_dex_file->Size(),
-                                  location->ToModifiedUtf8(),
-                                  0u,
-                                  nullptr,
-                                  nullptr);
+  std::unique_ptr<DexFile> dex_file(new DexFile(old_dex_file->Begin(),
+                                                old_dex_file->Size(),
+                                                location->ToModifiedUtf8(),
+                                                0u,
+                                                nullptr,
+                                                nullptr));
   {
     WriterMutexLock mu(soa.Self(), *class_linker->DexLock());
     // Check that inserting with a UTF16 name works.