Remove calls to deprecated V8 APIs in NpV8Object.cpp

R=jochen@chromium.org
BUG=324225

Review URL: https://codereview.chromium.org/95253002

git-svn-id: svn://svn.chromium.org/blink/trunk@162847 bbb929c8-8fbe-4397-9dbb-9b2b20218538