Ensure that privates are private.

      - Remove JS code injection functionality from UtilsNativeHandler.
- Ensure that utils.expose only exposes public properties.
- Prevent privates from getting poisoned via arbitrary constructor invocations.
- Prevent privates from leaking through prototypes.

BUG=603748

Review URL: https://codereview.chromium.org/1903303002

Cr-Commit-Position: refs/heads/master@{#389292}