Update the GN reference.md documentation.

TBR=dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2656873002
Cr-Original-Commit-Position: refs/heads/master@{#446110}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8598442a712230a26347c7adcfc95cbb8e95a46f
1 file changed
tree: 06d28efa97c074e0243bd498239fb1ee6b5107f8
  1. tools/