DevTools: Do not include thrown property descriptors in object preview.

Minor clean-up.

BUG=
R=pfeldman

Review URL: https://codereview.chromium.org/40093002

git-svn-id: svn://svn.chromium.org/blink/trunk@160715 bbb929c8-8fbe-4397-9dbb-9b2b20218538