DevTools: Wrong result of Runtime.getProperties call if Object.prototype has user-defined properties.

Use { __proto__: null } objects in InjectedScript.js so that to avoid interfering with user-defined Object.prototype.* values.

BUG=167400
R=yurys

Review URL: https://codereview.chromium.org/138233002

git-svn-id: svn://svn.chromium.org/blink/trunk@165465 bbb929c8-8fbe-4397-9dbb-9b2b20218538