Remove UnsafePersistent from V8NPTemplateMap + V8PerContextData.

(Change depends on 180363004.)

R=dcarney@chromium.org
BUG=276323

Review URL: https://codereview.chromium.org/206363002

git-svn-id: svn://svn.chromium.org/blink/trunk@171292 bbb929c8-8fbe-4397-9dbb-9b2b20218538