[gold] Make 'U' keep going to next biggest diff
[gold] Make 'U' keep going to next biggest diff

This also allows users to go backwards with 'Y'
and see where the selected pixel ranks as far
as most different.

Bug: skia:
Change-Id: Ieafca0b032f441bf72fb16fd64e90b2bed56cfd0
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/211407
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Reviewed-by: Nathaniel Nifong <1ce561679465ea4956c6210637580cd7ba456816@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot