Change UnsafePersistent<.> to Eternal<.> in V8PerIsolateData.
This is split out from 206533004, as it's largely unrelated to
the remaining changes there.

BUG=

Review URL: https://codereview.chromium.org/212793013

git-svn-id: svn://svn.chromium.org/blink/trunk@170196 bbb929c8-8fbe-4397-9dbb-9b2b20218538