If you don't build local desktop builds of Chrome OS Chrome (i.e. GN target_os="chromeos"), you can stop reading.
Chrome OS builds of Chrome now require additional dependencies to be synced before building. You can get these by adding "chromeos" to a target_os list in your .gclient file. This configuration has been supported for a while but has not been a strict requirement for desktop builds until now.
All Chrome OS builders, the chromeos-chrome ebuild, and the SimpleChrome workflow already have this configuration applied, so no changes are necessary in those cases.