[GN] move gn doc to proper place

R=dpranke@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/1305383008

Cr-Original-Commit-Position: refs/heads/master@{#346519}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fe59edcf12d785724ab7bf12e7864452724e0c59
2 files changed
tree: d33dda5ad6106070eb5450ea7458df1aa92ec973
  1. tools/