Don't panic when tearing down a WebGL context and the thead is unreachable.

This avoids some cascading double-panics when there are errors in the WebGL rendering thread.

components/script/dom/webglrenderingcontext.rs

@@ -1193,7 +1193,7 @@ impl WebGLRenderingContext {
 
 impl Drop for WebGLRenderingContext {
     fn drop(&mut self) {
-        self.webgl_sender.send_remove().unwrap();
+        let _ = self.webgl_sender.send_remove();
     }
 }