Remove ambiguous operator= on AutoCFRelease.

The operator= is ambiguous as to the state of the reference count.
Until we have rvalue assignment operators, just use 'reset' as the name
of the reference stealing assignment to AutoCFRelease.

R=reed@google.com, mtklein@google.com

Author: bungeman@google.com

Review URL: https://codereview.chromium.org/400453002