Replace V8ScriptInstance with SharedPersistent

V8ScriptInstance is a ref counted wrapper around a ScopedPersistent which is
identical to a SharedPersistent. We can remove it and replace the one usage
with that.

I also renamed the methods related to ScriptInstance to more accurately reflect
their purpose: to create a JS wrapper around a Plugin.

Review URL: https://codereview.chromium.org/22292006

git-svn-id: svn://svn.chromium.org/blink/trunk@159653 bbb929c8-8fbe-4397-9dbb-9b2b20218538