We recently spent a while diagnosing an issue with a patch that we were not able to reproduce locally or on the trybots. It turns out that it was a problem with necessary files not getting packaged after compilation, which isn't done locally or on the trybots.

The fix was a one line addition (https://chromium-review.googlesource.com/c/453099/), but diagnosing it was a pain since the error message gave no indication that this could be an issue with the build infrastructure (we thought it was an issue with some files not being built):

[7364:7468:0310/121800.244:1941125:ERROR:mojo_context_state.cc(199)] Failed to fetch source for module "gpu/ipc/common/mailbox_holder.mojom"
[7364:7468:0310/121800.244:1941125:ERROR:mojo_context_state.cc(199)] Failed to fetch source for module "gpu/ipc/common/sync_token.mojom"

This also seems to have happened in the past (https://chromium-review.googlesource.com/c/414565/), so something should probably be done to either:

- Prevent this from happening at all, e.g. packaging all of gen/, although I don't know how much that would affect size
- Make the error message hint at possible solutions, e.g. adding "Is the file in one of the directories included in build/scripts/slave/zip_build.py#_MojomFiles?" to the end.