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>

src/dawn_native/opengl/PipelineLayoutGL.cpp

@@ -54,7 +54,12 @@ namespace dawn_native { namespace opengl {
                         ssboIndex++;
                         break;
 
-                    // TODO(shaobo.yan@intel.com): Implement dynamic buffer offset
+                    case dawn::BindingType::StorageTexture:
+                    case dawn::BindingType::ReadonlyStorageBuffer:
+                        UNREACHABLE();
+                        break;
+
+                        // TODO(shaobo.yan@intel.com): Implement dynamic buffer offset
                 }
             }
         }