Add RefPtr constructor that takes a reference

This constructor is explicit, thus avoiding accidental ref()-ing, but still
avoids unnecessary use of the & operator in calling code.

R=ch.dumez@samsung.com, eseidel, esprehn, tkent

Review URL: https://codereview.chromium.org/27059003

git-svn-id: svn://svn.chromium.org/blink/trunk@159678 bbb929c8-8fbe-4397-9dbb-9b2b20218538