Add `privacy.network.webRTCMultipleRoutesEnabled` to extensions API.

Expose the preference for WebRTC's routing to extensions' privacy API. Disabling this preference prevents local IPs to be leaked when using WebRTC.

BUG=457629

Review URL: https://codereview.chromium.org/940893003

Cr-Commit-Position: refs/heads/master@{#317066}
4 files changed