DevTools: Injected __commandLineAPI property should not be enumerable.

Evaling "keys(window).indexOf('__commandLineAPI')" in console should return -1.

// not directly related
BUG=415578
R=pfeldman

Review URL: https://codereview.chromium.org/580243006

git-svn-id: svn://svn.chromium.org/blink/trunk@182316 bbb929c8-8fbe-4397-9dbb-9b2b20218538