在webview嵌套html页面调用navigator.mediaDevices.getUserMedia()不存在

需求是在app里面webview嵌套html页面，在html页面进行webrtc推流，目前调用navigator.mediaDevices.getUserMedia()方法报错，打印navigator是一个plus包裹的对象。
如下

{  
    "plus": {  
        "isReady": true,  
        "tools": Object{...},  
        "bridge": Object{...},  
        "obj": Object{...},  
        "accelerometer": Object{...},  
        "audio": Object{...},  
        "barcode": Object{...},  
        "cache": Object{...},  
        "camera": Object{...},  
        "contacts": Object{...},  
        "downloader": Object{...},  
        "gallery": Object{...},  
        "geolocation": Object{...},  
        "io": Object{...},  
        "maps": Object{...},  
        "messaging": Object{...},

怎么使用navigator.mediaDevices.getUserMedia()？