Add v8 Function Converter ToV8 to gin

This changes adds the ToV8 converter to gin for v8::Function
This is used by electron internally and could be helpful to other
embedders as well.

Change-Id: I19067d1f5cb119a58f1a8574dd9f232b72b328bf
Reviewed-on: https://chromium-review.googlesource.com/c/1461826
Auto-Submit: Nitish Sakhawalkar <nsakhawalkar@slack-corp.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630782}

• AUTHORS[diff]
• gin/converter.cc[diff]
• gin/converter.h[diff]