[WebGPU] Default GPUVertexInputDescriptor indexFormat to uint32

This CL makes sure GPUVertexInputDescriptor indexFormat defaults
to uint32 as spec'ed in https://github.com/gpuweb/gpuweb/pull/270

Bug: 877147
Change-Id: I625298abd78794a172d9296a511747b4363dbbe0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1641263
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665542}

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

@@ -5,7 +5,7 @@
 // https://github.com/gpuweb/gpuweb/blob/master/design/sketch.webidl
 
 dictionary GPUVertexInputDescriptor {
-    required GPUIndexFormat indexFormat;
+    GPUIndexFormat indexFormat = "uint32";
     // TODO(crbug.com/951629): Make this a sequence of nullables.
     object vertexBuffers; // We validate this is an array of nullable GPUVertexBufferDescriptor
 };