Add documentation about no sources for GN guide.

This just adds another consideration for source sets versus static libraries.

TBR=dpranke

Review-Url: https://codereview.chromium.org/2082103002
Cr-Original-Commit-Position: refs/heads/master@{#401103}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 120e860e372e1654422696dcb8ab49a22e438d78
1 file changed
tree: fc177c4c330151c2c6113a1e8c1605bc681e27be
  1. tools/