remove UnsafePersistent::deprecatedHandle()

v8::ReturnValue accepts v8::Persistent, so this is not needed now.

BUG=None

Review URL: https://codereview.chromium.org/120413004

git-svn-id: svn://svn.chromium.org/blink/trunk@164327 bbb929c8-8fbe-4397-9dbb-9b2b20218538