[viewer] Add color histogram

Change-Id: Ib3a4041a40de5925ebf98b325122acb3261a9d92
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/479436
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
2 files changed