Update compositing benchmark to use image-set API

Change-Id: I73de5cb894e4084ca86e43c2918a1a9fe656b11d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209323
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
1 file changed