Replace UnsafePersistent with v8::PersistentValueVector.

(Depends on https://codereview.chromium.org/216973002)

BUG=

Review URL: https://codereview.chromium.org/217193002

git-svn-id: svn://svn.chromium.org/blink/trunk@171264 bbb929c8-8fbe-4397-9dbb-9b2b20218538