Update Building NW.js.md

[rebeccasf]

This is to ensure the documentation is update to support latest branch to avoid branch confusions among all the repos required in building nw.js.

Since chromium has started to shift focus to GN, GYP variables are now required explicit export in order to be able to generate ninja build files with GYP for Node.

Also, the gyp_chromium.py was written in Python2 syntax, hence any host that has Python3 set as default will have issue running the command.

[rebeccasf]

@rogerwang I would like to request your review on this doc update. Thank you.

[corwin-of-amber]

In fact it was mentioned in the past (I believe by @TheJaredWilcurt) that step 1 was outdated, namely that manual clone of node and v8 is anachronistic and that these should be handled by DEPS file in nw.js. I am sorry for not remembering the exact details. Am I striking something?

[div72]

The link in Build Faster for Linux should be changed to https://chromium.googlesource.com/chromium/src/+/HEAD/docs/linux/build_instructions.md#faster-builds.

[ayushmanchhabra]

I'm going to close this since this PR doesn't look like it's making any progress. I'm going to open a bunch of PRs to update the docs.