Add ComparisonSampler binding type and validation tests

Bug: dawn:367
Change-Id: Iba1d3d03f6247a356b6f3fabfe7a7ba3c0753171
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/18423
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

src/dawn_native/Sampler.cpp

@@ -80,6 +80,10 @@ namespace dawn_native {
         return new SamplerBase(device, ObjectBase::kError);
     }
 
+    bool SamplerBase::HasCompareFunction() const {
+        return mCompareFunction != wgpu::CompareFunction::Undefined;
+    }
+
     size_t SamplerBase::HashFunc::operator()(const SamplerBase* module) const {
         size_t hash = 0;