v8: make ObjectMirror handle proxy objects
Refs: https://github.com/nodejs/node/issues/10406
@eugeneo @ofrobots Should I send this upstream? IIUC, mirrors are being phased out?
CI: https://ci.nodejs.org/job/node-test-pull-request/5531/ CI: https://ci.nodejs.org/job/node-test-commit-v8-linux/476/