Initial implementation of GPUCommandEncoder

Added WebIDL bindings for `GPUCommandEncoder`, `GPUCommandBuffer`, `GPUComputePassEncoder`, `GPUProgrammablePassEncoder`.
Implemented the `beginComputePass`, `copyBufferToBuffer` and `finish` functions of `GPUCommandEncoder`.
Implemented the `createCommandEncoder` function of `GPUDevice`.

components/script/dom/gpucommandbuffer.rs

@@ -0,0 +1,58 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use crate::dom::bindings::cell::DomRefCell;
+use crate::dom::bindings::codegen::Bindings::GPUCommandBufferBinding::{
+    self, GPUCommandBufferMethods,
+};
+use crate::dom::bindings::reflector::{reflect_dom_object, Reflector};
+use crate::dom::bindings::root::DomRoot;
+use crate::dom::bindings::str::DOMString;
+use crate::dom::globalscope::GlobalScope;
+use dom_struct::dom_struct;
+use webgpu::{WebGPU, WebGPUCommandBuffer};
+
+#[dom_struct]
+pub struct GPUCommandBuffer {
+    reflector_: Reflector,
+    #[ignore_malloc_size_of = "defined in webgpu"]
+    channel: WebGPU,
+    label: DomRefCell<Option<DOMString>>,
+    command_buffer: WebGPUCommandBuffer,
+}
+
+impl GPUCommandBuffer {
+    pub fn new_inherited(channel: WebGPU, command_buffer: WebGPUCommandBuffer) -> GPUCommandBuffer {
+        GPUCommandBuffer {
+            channel,
+            reflector_: Reflector::new(),
+            label: DomRefCell::new(None),
+            command_buffer,
+        }
+    }
+
+    pub fn new(
+        global: &GlobalScope,
+        channel: WebGPU,
+        command_buffer: WebGPUCommandBuffer,
+    ) -> DomRoot<GPUCommandBuffer> {
+        reflect_dom_object(
+            Box::new(GPUCommandBuffer::new_inherited(channel, command_buffer)),
+            global,
+            GPUCommandBufferBinding::Wrap,
+        )
+    }
+}
+
+impl GPUCommandBufferMethods for GPUCommandBuffer {
+    /// https://gpuweb.github.io/gpuweb/#dom-gpuobjectbase-label
+    fn GetLabel(&self) -> Option<DOMString> {
+        self.label.borrow().clone()
+    }
+
+    /// https://gpuweb.github.io/gpuweb/#dom-gpuobjectbase-label
+    fn SetLabel(&self, value: Option<DOMString>) {
+        *self.label.borrow_mut() = value;
+    }
+}