PrivateScriptRunner should use GetOwnPropertyDescriptor in V8 APIs

This CL removes hand-written implementation for Object.getOwnPropertyDescriptor
and instead uses a V8 API.

BUG=341031

Review URL: https://codereview.chromium.org/387343004

git-svn-id: svn://svn.chromium.org/blink/trunk@178064 bbb929c8-8fbe-4397-9dbb-9b2b20218538