Remove calls to deprecated V8 APIs in ScriptWrappable.h, V8StringResource.h, UnsafePersistent.h

BUG=324225
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/94543002

git-svn-id: svn://svn.chromium.org/blink/trunk@162826 bbb929c8-8fbe-4397-9dbb-9b2b20218538