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: svn://svn.chromium.org/blink/trunk@158279 bbb929c8-8fbe-4397-9dbb-9b2b20218538