Add documentation for GN's GYP generation.

BUG=
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/35933003

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0ff08a372f26e388e35d4dc78c6760c6659bab15
2 files changed
tree: c8f37861bcd2f0d79677e4f295ef49a1d2537df0
  1. tools/