Move Tracer into shared code.

Nothing here was SkVM-specific, except for the reference to a
`SkVMDebugTrace*`. The class now targets the vector inside the
debug trace, and would be safe to share with RP.

Change-Id: I82ac24993fc834ef16a6844372939e2097138bb6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/660558
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
6 files changed