Cut down repeated code in RefPtr by using copy/move-and-swap for assignment operators

Extract from WebKit patch by Darin Adler <darin@apple.com> r154519.

RetainPtr is not modified because it requires some preliminary refactoring before that.

Review URL: https://chromiumcodereview.appspot.com/23666004

git-svn-id: svn://svn.chromium.org/blink/trunk@156871 bbb929c8-8fbe-4397-9dbb-9b2b20218538