See e.g. https://code.google.com/p/chromium/codesearch#chromium/src/out/Debug/gen/chrome/common/resource_usage_reporter.mojom.cc&sq=package:chromium

It's good practice for autogenerated files to say at the top that they're autogenerated, and ideally also hint at what autogenerated them ("by build rule foo", "by tool toolname", etc).