[WebGPU] Make GPURenderPipeline.fragmentStage optional

Following https://dawn-review.googlesource.com/c/dawn/+/9641, a
GPURenderPipeline fragmentStage is not required anymore.
Note that it will still raise a validation error as vertex-only render
pipelines are not supported yet in Dawn.

Bug: 877147
Change-Id: I2ccf88974fa011a6ba3a45d229e36e40e23c5dc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730057Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#683522}

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

@@ -221,11 +221,13 @@ GPURenderPipeline* GPURenderPipeline::Create(
   OwnedPipelineStageDescriptor vertex_stage_info =
       AsDawnType(webgpu_desc->vertexStage());
   dawn_desc.vertexStage = &std::get<0>(vertex_stage_info);
-
-  // TODO(crbug.com/dawn/136): Support vertex-only pipelines.
-  OwnedPipelineStageDescriptor fragment_stage_info =
-      AsDawnType(webgpu_desc->fragmentStage());
-  dawn_desc.fragmentStage = &std::get<0>(fragment_stage_info);
+  OwnedPipelineStageDescriptor fragment_stage_info;
+  if (webgpu_desc->hasFragmentStage()) {
+    fragment_stage_info = AsDawnType(webgpu_desc->fragmentStage());
+    dawn_desc.fragmentStage = &std::get<0>(fragment_stage_info);
+  } else {
+    dawn_desc.fragmentStage = nullptr;
+  }
 
   // TODO(crbug.com/dawn/131): Update Dawn to match WebGPU vertex input
   v8::Isolate* isolate = script_state->GetIsolate();

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

@@ -6,8 +6,7 @@
 
 dictionary GPURenderPipelineDescriptor : GPUPipelineDescriptorBase {
     required GPUPipelineStageDescriptor vertexStage;
-    // TODO(crbug.com/dawn/136): Support vertex-only pipelines.
-    required GPUPipelineStageDescriptor fragmentStage;
+    GPUPipelineStageDescriptor fragmentStage;
 
     required GPUPrimitiveTopology primitiveTopology;
     GPURasterizationStateDescriptor rasterizationState;