Combine SkVM/SkRP debug traces into a single class.

It wasn't really beneficial to have two classes here at all.
`SkVMDebugTrace` is now renamed to `DebugTracePriv`, and both
SkVM and SkRP now share the same class.

Change-Id: Id5c97c7efdf769d3cbb3495b3b3bec1cb80bd0a9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/661060
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
37 files changed