[WebGPU] Enable the support for readonly storage buffer enum

Readonly storage buffer is fully implemented in Dawn. We should
enable the support for related enum.

BUG=dawn:180

Change-Id: I13ba9b61757b8c1e7f612c93f34947c510663cbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1957744Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Yunchao He <yunchao.he@intel.com>
Cr-Commit-Position: refs/heads/master@{#723432}

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

@@ -23,6 +23,9 @@ 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

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