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}
(cherry picked from commit 77e0fbe12e32b16d5c1d7c0380b45fde363004b2)

Review URL: https://codereview.chromium.org/1938123002 .

Cr-Commit-Position: refs/branch-heads/2704@{#337}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}
18 files changed