Adds the ability to serialize sampler objects.
Adds serialization of sampler objects to serializeContext method so
that capture replay regression testing now compares the states of
samplers too.

Bug: angleproject:4817
Change-Id: Ic32d442eac9aa3afc5c9eea577522fe7054a755a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2315048
Commit-Queue: Manh Nguyen <nguyenmh@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>