Add missing descriptors that are present in WebGPU.

These are the CommandEncoder, ComputePass and CommandBuffer descriptors
that contains nothing but a debug name for now but are important for
later extensibility. Defaults are added so the C++ API doesn't require
the descriptors to be passed as arguments.

Also renames variables named "info" for RenderPassDescriptor to
"descriptor" as is now the standard in the codebase.

BUG=dawn:22

Change-Id: I9de4cfbbce952d01fb79ed1d9f34825a6fa174f9
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/8686
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>

examples/CHelloTriangle.cpp

@@ -141,7 +141,7 @@ void frame() {
     }
     DawnCommandBuffer commands;
     {
-        DawnCommandEncoder encoder = dawnDeviceCreateCommandEncoder(device);
+        DawnCommandEncoder encoder = dawnDeviceCreateCommandEncoder(device, nullptr);
 
         DawnRenderPassEncoder pass = dawnCommandEncoderBeginRenderPass(encoder, &renderpassInfo);
         dawnRenderPassEncoderSetPipeline(pass, pipeline);
@@ -149,7 +149,7 @@ void frame() {
         dawnRenderPassEncoderEndPass(pass);
         dawnRenderPassEncoderRelease(pass);
 
-        commands = dawnCommandEncoderFinish(encoder);
+        commands = dawnCommandEncoderFinish(encoder, nullptr);
         dawnCommandEncoderRelease(encoder);
     }