Add a setting to disable SkVM variable traces.

This will allow the visualizer to see line-number information without
incurring the code bloat caused by trace_var.

For reference, Commutative.skvm with no debug traces:
https://osscs.corp.google.com/skia/skia/+/main:tests/sksl/runtime/Commutative.skvm;drc=c809e5ba9fc9bc67291d088c448ed0057371b760

Change-Id: If98b5e102571d7fe06a1653d199e3ae035b3cb78
Bug: skia:12692
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/478417
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Julia Lavrova <jlavrova@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
5 files changed