Use webxr IPC to get a WebXR device registry to each script thread

components/canvas_traits/Cargo.toml

@@ -25,6 +25,7 @@ pixels = {path = "../pixels"}
 serde = "1.0"
 serde_bytes = "0.10"
 servo_config = {path = "../config"}
+typetag = "0.1"
 webrender_api = {git = "https://github.com/servo/webrender", features = ["ipc"]}
 webvr_traits = {path = "../webvr_traits"}
 webxr-api = {git = "https://github.com/servo/webxr"}

components/canvas_traits/webgl.rs

@@ -177,6 +177,7 @@ impl WebGLMsgSender {
     }
 }
 
+#[typetag::serde]
 impl webxr_api::WebGLExternalImageApi for WebGLMsgSender {
     fn lock(&self) -> Result<(GLuint, Size2D<i32>, GLsync), webxr_api::Error> {
         let (sender, receiver) = webgl_channel().or(Err(webxr_api::Error::CommunicationError))?;