DevTools: Fix broken Watch Pane with "__proto__" expression.

Also did some clean up.

BUG=428302
R=eustas

Review URL: https://codereview.chromium.org/683363005

git-svn-id: svn://svn.chromium.org/blink/trunk@184596 bbb929c8-8fbe-4397-9dbb-9b2b20218538