Address a few TODOs in the gn config
Address a few TODOs in the gn config

- add direct dependant settings
- only compile mksnapshot on the host toolset

BUG=none
LOG=n
R=machenbach@chromium.org, brettw@chromium.org

Review URL: https://codereview.chromium.org/314073002

git-svn-id: f98b9d40cb02301bc76fa9e1b46ee668158567fd@21689 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh