Revert "Revert "Add DSL PossibleExpression & PossibleStatement""

This reverts commit 4903482a2a0c05de41aba597624967d0436b37bf.

Change-Id: Ie56583344897fcb2c072bcac4d97d582edeadf07
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/375660
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
9 files changed