Fixed DSL struct nesting check

The DSLParser struct nesting check didn't work, and on top of that
should have been part of the core DSL itself as opposed to being
relegated to the parser.

Change-Id: I680d0dc703309136970353204add14a6c9e03aa8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/445113
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
5 files changed