Once this CL lands and is included in Node.js, we can
- run `tools/node/fetch_deps.py <v8_path>` to fetch necessary deps.
- run `tools/node/build_gn.py <build mode> <v8_path> <out_dir>` to build.
- use new v8_monolith target in v8.gyp to call build_gn.py.

R=machenbach@chromium.org

Bug: v8:6105
Change-Id: I482bfddb40f77df62394a913335bd43627cc0c43
Reviewed-on: https://chromium-review.googlesource.com/792944Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50152}