GN GRD updates

[Tim Van der Lippe]

Hey everyone,

Yesterday we changed the way we are generating the DevTools GRD file: https://crrev.com/c/2843322 We now use GN metadata (https://gn.googlesource.com/gn/+/master/docs/reference.md#var_metadata) to declare GRD files in GN actions.

Therefore, each action in the dependency graph can declare files it contributes to the GRD. As such, the previous lists in `config/gni/all_devtools_modules.gni` and `config/gni/devtools_module_entrypoints.gni` are obsolete and have been removed in https://crrev.com/c/2844169

If you are adding/removing files, you now only need to update the debug/release lists in `config/gni/devtools_grd_files.gni`. The build will fail if you forget to add/remove files from this list.

If you have any questions, let us know.

Cheers,

Tim & Alex