Add WebGPU identity management

Issue: https://github.com/servo/servo/issues/24706#issuecomment-557891841

components/script/dom/identityhub.rs

@@ -0,0 +1,47 @@
+/* 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 webgpu::wgpu::{AdapterId, Backend, DeviceId, IdentityManager, SurfaceId};
+
+#[derive(Debug)]
+pub struct IdentityHub {
+    adapters: IdentityManager<AdapterId>,
+    devices: IdentityManager<DeviceId>,
+}
+
+impl IdentityHub {
+    fn new(backend: Backend) -> Self {
+        IdentityHub {
+            adapters: IdentityManager::new(backend),
+            devices: IdentityManager::new(backend),
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct Identities {
+    surface: IdentityManager<SurfaceId>,
+    hub: IdentityHub,
+}
+
+impl Identities {
+    pub fn new() -> Self {
+        let hub = if cfg!(any(target_os = "linux", target_os = "windows")) {
+            IdentityHub::new(Backend::Vulkan)
+        } else if cfg!(any(target_os = "ios", target_os = "macos")) {
+            IdentityHub::new(Backend::Metal)
+        } else {
+            IdentityHub::new(Backend::Empty)
+        };
+
+        Identities {
+            surface: IdentityManager::new(Backend::Empty),
+            hub,
+        }
+    }
+
+    pub fn create_adapter_id(&mut self) -> AdapterId {
+        self.hub.adapters.alloc()
+    }
+}