[gold] pagination-sk: Add CSS classes to pagination buttons.
[gold] pagination-sk: Add CSS classes to pagination buttons.

This will allow us to more easily reference the pagination buttons from tests.

Bug: skia:9525
Change-Id: I284a652103dd166750c87ae27d3882d9ff71bf29
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/251241
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Commit-Queue: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>

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