From discussion on gn-dev:

However, it also appears that android device / ios device / swarming need three different things, and so we're still working out what commonality does exist and what the best way to implement things is. It looks like data/data_deps by itself isn't quite good enough, and bundle_data probably isn't much better, and now we're exploring ways to get the runtime_deps exposed programmatically in the build itself (or, at least, indicate that we need the runtime_deps for a given target to be generated so that a python script can manipulate it as part of the build).