subscribe access “back” acamera with html5 on android phone

Hi,  When I use html5 'getUserMedia' API to access acamera on the
android£¨4.0£© phone£¬ it comes out "front camera", but I want to open "back
camera".  browser is chrome26. JavaScript code:
window.URL = window.URL || window.webkitURL || window.msURL
|| window.oURL;
// Normalizes navigator.getUserMedia
navigator.getUserMedia = navigator.getUserMedia
|| navigator.webkitGetUserMedia
|| navigator.mozGetUserMedia || navigator.msGetUserMedia;

var contraints = {
 /*   mandatory: {
    width: { min: 640 },
    height: { min: 480 }
  }, */
  optional: [
    { width: 650 },
    { width: { min: 640 }},
    { frameRate: 60 },
    { width: { max: 800 }},
    { facingMode: "environment" }
  ]
};
navigator.getUserMedia({video:contraints}, successsCallback, errorCallback);

I don't know how to access specific camera on android phone. Can you help
me, sorry ,My english is poor £¬thanks^_^