Works around a bug in `gn desc` where values can sometimes be reported
as strings when we would expect them to be lists. This was causing
issues with the migration to abseil, as scripts/export_targets.py
did not have a workaround for this bug.

Bug: angleproject:4873
Change-Id: I69994e2cd68a563110fcb98072a24f59f70a06d3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2380716
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>