[WebGPU] Add default for GPURenderPassColorAttachmentDescriptor.storeOp

Following WebGPU spec change at https://github.com/gpuweb/gpuweb/issues/376,
GPURenderPassColorAttachmentDescriptor.storeOp default should be "store".

Bug: 877147
Change-Id: I7dd5db4e3cfcb0ddc4dd132342d8bf145bb8f28e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715186Reviewed-by: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#691530}

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

@@ -9,5 +9,5 @@ dictionary GPURenderPassColorAttachmentDescriptor {
     GPUTextureView resolveTarget;
 
     required (GPULoadOp or GPUColor) loadValue;
-    required GPUStoreOp storeOp;
+    GPUStoreOp storeOp = "store";
 };