Have the bots validate BUILD.gn

Currently if a file is added to the build, but updating BUILD.gn is missed, the bots won't detect it. The issue will only be detected when downstream users that depend on gn attempt to integrate the change. We should probably have a bot that would catch this type of problem.

The issue names BUILD.gn but no bot or CI file. Start by locating the repository's bot and CI configuration and how BUILD.gn inputs are checked; done means a bot reliably reports when an added file is missing from BUILD.gn.

Written by the indexing model from the issue text.