Introduce PassRefPtr::AdoptRefTag and use it in the adoption constructor

Introduce PassRefPtr::AdoptRefTag and use it in the adoption constructor instead of bool argument.
Rationals:
- makes code more consistent, since having this kind of tags is a pattern in blink
- makes code more clear: it does not have an explanation comment any more.

Review URL: https://codereview.chromium.org/70383008

git-svn-id: svn://svn.chromium.org/blink/trunk@162123 bbb929c8-8fbe-4397-9dbb-9b2b20218538