[canvaskit] Fix memory leak in paragraph bindings

This also renames some variables to be consistent.

See https://github.com/flutter/flutter/issues/56938

Change-Id: I0db66fa334d5f5efc1f94fff1a367a32e03611ca
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/289778
Reviewed-by: Nathaniel Nifong <nifong@google.com>
4 files changed