DevTools: Dont allow editing internal object properties in console.

Internal properties inherently are not writeable, thus they should behave so in the frontend.

R=vsevik

Review URL: https://codereview.chromium.org/323313005

git-svn-id: svn://svn.chromium.org/blink/trunk@176184 bbb929c8-8fbe-4397-9dbb-9b2b20218538