add RP comparison for SkVM_Overhead

Looks like ~50ns overhead for RP vs ~14,000ns for SkVM.

Change-Id: I85ef73d3387657b14615fcfa5cfd9df5c2325343
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/223302
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed