webgpu: Use WGPU poller thread for poll_all_devices (servo#32266) (we…

…bgpu)

{"fail_fast": false, "matrix": [{"name": "WebGPU CTS", "workflow": "linux", "wpt_layout": "2020", "profile": "production", "unit_tests": false, "wpt_tests_to_run": "_webgpu"}]}