Add missing WebGPU enums and defaulted members.

These additions to dawn.json don't require changes to users of the C++
API so they are done now to prepare for webgpu.h. Validation is added
that forbids using the new enum values and non-default values for
members until they are implemented.

BUG=dawn:22

Change-Id: Ie6f3384243d6874e7aca4b94733409755a066c93
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/8862
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>

examples/CHelloTriangle.cpp

@@ -112,6 +112,8 @@ void init() {
         descriptor.rasterizationState = &rasterizationState;
 
         descriptor.primitiveTopology = DAWN_PRIMITIVE_TOPOLOGY_TRIANGLE_LIST;
+        descriptor.sampleMask = 0xFFFFFFFF;
+        descriptor.alphaToCoverageEnabled = false;
 
         descriptor.depthStencilState = nullptr;