v8: make ObjectMirror handle proxy objects

[bnoordhuis]

Refs: #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/

[ofrobots]

Thanks for fixing. IMO, this should be contributed upstream. The mirrors code is slated to go away soonish, but the code does exists upstream for now. /cc @hashseed.

[TimothyGu]

Ping. Any updates?

[bnoordhuis]

I never got around to it because V8's debugger tests were in a severe state of flux. I'll see if I can get around to it this week, I have more patches that I need to upstream.

[targos]

@bnoordhuis Knowing that the Debug API is going away, I suppose upstreaming this doesn't make much sense anymore.

[bnoordhuis]

Ah, I rather forgot about this PR. Are you suggesting I close it or that I merge it without upstreaming? I suppose it's still relevant in v6.x.

[targos]

I'm suggesting to merge in v6.x without upstreaming.

[bnoordhuis]

Opened #14343 for v6.x, input welcome.