Currently Viewer fires unbalanced load events on backend change.
Instead of calling setupCurrentSlide() with an invalid prevSlide and
always forcing a slide load, pass a |force| argument on backend change.
Change-Id: I146b42331f5aa9721d4f407b81c13f4512d67d61
Reviewed-on: https://skia-review.googlesource.com/94960
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>