Update all Sampler handles to type SamplerID, preparing for midframe
capture of sampler.

Bug: angleproject:3804
Change-Id: I9337919f97d61e28718987612d642c569b90246a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1761780
Commit-Queue: Jiacheng Lu <lujc@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>