Fix clang/win build after https://codereview.chromium.org/1471123002

BUG=560836
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/1466423006 .

Cr-Commit-Position: refs/heads/master@{#361340}

mojo/edk/system/parent_token_serializer_win.h

@@ -29,7 +29,7 @@ class MOJO_SYSTEM_IMPL_EXPORT ParentTokenSerializer
   ParentTokenSerializer(HANDLE child_process, ScopedPlatformHandle pipe);
 
  private:
-  virtual ~ParentTokenSerializer();
+  virtual ~ParentTokenSerializer() override;
 
   void RegisterIOHandler();
   void BeginRead();