Initial implementation of WebGPU API

components/script/dom/webidls/GPUAdapter.webidl

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+/* 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/. */
+
+// https://gpuweb.github.io/gpuweb/#gpuadapter
+[Exposed=(Window, DedicatedWorker), Pref="dom.webgpu.enabled"]
+interface GPUAdapter {
+    readonly attribute DOMString name;
+    readonly attribute object extensions;
+    //readonly attribute GPULimits limits; Don’t expose higher limits for now.
+
+    // May reject with DOMException  // TODO: DOMException("OperationError")?
+    // Promise<GPUDevice> requestDevice(optional GPUDeviceDescriptor descriptor = {});
+};