Make ScopedPersistent::deprecatedGet private.

All the callers outside ScopedPersistent have been removed.

This also renames it to deprecatedHandle.

BUG=236290

Review URL: https://chromiumcodereview.appspot.com/19697003

git-svn-id: svn://svn.chromium.org/blink/trunk@154488 bbb929c8-8fbe-4397-9dbb-9b2b20218538