GN bootstrap fixes:

- add missing base/threading/thread.cc to compile targets
- add --gn-gen-args for passing custom --args to gn gen
  (when building GN with itself)
- allow overriding use_cups

BUG=551343

Review URL: https://codereview.chromium.org/1415343008

Cr-Original-Commit-Position: refs/heads/master@{#358090}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ae483ee1eccc94ab232a6f6e91874d852edd0942
1 file changed
tree: 62f0c6a883ba3fe986f3bc6d5228cd838d2185fb
  1. tools/