Refactor: replace SamplerType with ShaderType BUG=angleproject:1987 Change-Id: I189e1606bd966eb369e8192a6866c8e90810e937 Reviewed-on: https://chromium-review.googlesource.com/802956 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yunchao He <yunchao.he@intel.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>