Fix modifier flags on function parameters in description().

Previously, these were silently dropped. We didn't have any DSL tests
for function-parameter modifiers (which is where `description()`
usually gets tested).

Change-Id: I3323537a76dfe536227b01bc9dca8665850aa8c2
Bug: skia:13775
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/585961
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
4 files changed