Cache validate samplers result. This gives ~23% increase in the validation-only draw call perf test. BUG=angleproject:959 Change-Id: I384a5c4fbb1c2cd47483bd7cf4bc1d39447a99bc Reviewed-on: https://chromium-review.googlesource.com/267750 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Brandon Jones <bajones@chromium.org>