PropertyCallbackInfo::This() deleted in v8 14.6

[zawata]

I've been updating our addon to electron 41(v8 14.6.202) and it fails to compile for 2 reasons:

1. v8::PropertyCallbackInfo::This() has been removed (commit, chromium-issue)
2. v8::PropertyCallbackInfo::Holder() has been renamed to HolderV2 (chromium-issue)

I'm not an expert on this subject and the fact that it's 2 separate deprecations occurring at the same time is confusing me as to the proper way to go. It seems like Holder was deprecated in favor of This which was deprecated in favor of HolderV2.

Our addon can switch from This to HolderV2 trivially but cloudflare has taken issue with this change. I've made a fork to naively delete nan::PropertyCallbackInfo::This and make nan::PropertyCallbackInfo::Holder() call v8::PropertyCallbackInfo::HolderV2() but I don't know if this covers all the expected cases, hence the issue.