Source handling needs to cope with new 'gcs' type

Our existing source handling script has cipd and git sources, but the Electron 31 release is throwing an error with something about gcs sources, whatever that is.

==> Starting prepare()...
Putting together electron sources
Skipping src/third_party/pyyaml because of unmet condition checkout_pyyaml and process_deps
Ignoring src/third_party/squirrel.mac
Ignoring src/third_party/squirrel.mac/vendor/ReactiveObjC
Ignoring src/third_party/squirrel.mac/vendor/Mantle
Fetching recursedep src
Traceback (most recent call last):
  File "/build/electron31/src/makepkg-source-roller.py", line 350, in <module>
    git_deps, gargs, cipd_deps, reverse_map = parse_deps(deps_path)
                                              ^^^^^^^^^^^^^^^^^^^^^
  File "/build/electron31/src/makepkg-source-roller.py", line 151, in parse_deps
    dep_deps, dep_gclient_gn_args, dep_cipd_deps, _ = parse_deps(
                                                      ^^^^^^^^^^^
  File "/build/electron31/src/makepkg-source-roller.py", line 118, in parse_deps
    raise Exception(f"Unknown DEP {dep_name} = {dep_value}")
Exception: Unknown DEP src/third_party/js_code_coverage = {'dep_type': 'gcs', 'condition': 'checkout_js_coverage_modules and non_git_source', 'bucket': 'chromium-nodejs', 'objects': [{'object_name': 'js_code_coverage/d538975c93eefc7bafd599b50f867e90c1ef17f3', 'sha256sum': '646bb00ced0a930b2eb1e4dbcfac18ebbb8f889bb80599e0254d9d6505427914', 'siz
e_bytes': 1469185, 'generation': 1657780123604338}]}

@kxxt Does this ring any bells for you or sound like something you know how to fix? This particular dep sounds like something we shouldn't need for a step we could skip anyway (we don't need coverage info during our builds) but I don't know if there are more deps lurking to show up after we get past this one or other 'gcs' sources we'll get hung up on.