[WebGPU] Remove more unnecessary nullables

Following WebGPU spec change at https://github.com/gpuweb/gpuweb/pull/388,
this CL removes some unnecessary nullables, specifically in resolveTarget
from GPURenderPassColorAttachmentDescriptor.

Bug: 877147
Change-Id: Ib24b76ba65c22273b2d7bfef50ef8cc22e09daad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1748666
Auto-Submit: François Beaufort <beaufort.francois@gmail.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686217}

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

@@ -6,7 +6,7 @@
 
 dictionary GPURenderPassColorAttachmentDescriptor {
     required GPUTextureView attachment;
-    GPUTextureView? resolveTarget = null;
+    GPUTextureView resolveTarget;
 
     required (GPULoadOp or GPUColor) loadValue;
     required GPUStoreOp storeOp;