Make the copy assignment operator of PassRefPtr private.
Make the copy assignment operator of PassRefPtr private.

Rationale
1. It is not used.
2. This implementation is not correct.

This CL doen't remove the implementation because COMPILE_ASSERT is still useful.

BUG=N/A

Review URL: https://chromiumcodereview.appspot.com/24152014

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@158279 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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