[WebGPU] Remove support for readonly storage buffer enum

We should not expose this enum until it is fully implemented
in Dawn. This is temporary to not have the tests crash while
we incrementally implement the feature.

Bug: dawn:180
Change-Id: Iea128b20a792dfae926ed64bc5b03feb41fc5811
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899195
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Yunchao He <yunchao.he@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713074}

third_party/blink/renderer/modules/webgpu/dawn_conversions.cc

@@ -23,9 +23,6 @@ WGPUBindingType AsDawnEnum<WGPUBindingType>(const WTF::String& webgpu_enum) {
   if (webgpu_enum == "storage-buffer") {
     return WGPUBindingType_StorageBuffer;
   }
-  if (webgpu_enum == "readonly-storage-buffer") {
-    return WGPUBindingType_ReadonlyStorageBuffer;
-  }
   if (webgpu_enum == "sampler") {
     return WGPUBindingType_Sampler;
   }

third_party/blink/renderer/modules/webgpu/gpu_bind_group_layout_binding.idl

@@ -17,7 +17,6 @@ dictionary GPUBindGroupLayoutBinding {
 enum GPUBindingType {
     "uniform-buffer",
     "storage-buffer",
-    "readonly-storage-buffer",
     "sampler",
     "sampled-texture",
     "storage-texture",