[WebGPU] Set default value for vertexInput in GPURenderPipelineDescriptor

This CL simply set the default value of vertexInput in
GPURenderPipelineDescriptor to an empty dictionary as specified in
https://gpuweb.github.io/gpuweb/#dom-gpurenderpipelinedescriptor-vertexinput

Bug: 877147
Change-Id: I3e8d8fe42217a563b133767319d9695d21683186
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824884
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Auto-Submit: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700143}

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

@@ -12,7 +12,7 @@ dictionary GPURenderPipelineDescriptor : GPUPipelineDescriptorBase {
     GPURasterizationStateDescriptor rasterizationState;
     required sequence<GPUColorStateDescriptor> colorStates;
     GPUDepthStencilStateDescriptor depthStencilState;
-    GPUVertexInputDescriptor vertexInput;
+    GPUVertexInputDescriptor vertexInput = {};
 
     unsigned long sampleCount = 1;