Implement trace_var opcode.

This writes an entry to the trace buffer every time a slot value is
changed.

Change-Id: Iac3912be71ad654f70a7158e306e0643086c6cb0
Bug: skia:12614
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/469179
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
10 files changed