Use placement new to initialize the persistent in V8NPObject

We just allocate some chunk of memory and reinterpret_cast it to
V8NPObject. We still need to properly construct the persistent inside of
it.

BUG=324225
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/96613005

git-svn-id: svn://svn.chromium.org/blink/trunk@162905 bbb929c8-8fbe-4397-9dbb-9b2b20218538