Oilpan: Add implicit conversion from Persistent<T> to PassRefPtr<U>.

We can avoid to add unnecessary .get() during the Oilpan transition period.

BUG=

Review URL: https://codereview.chromium.org/288593005

git-svn-id: svn://svn.chromium.org/blink/trunk@174068 bbb929c8-8fbe-4397-9dbb-9b2b20218538