Added devicePixelRatio support (fixes #10964)

[vegetableman]

fixes #10964

Example:-

var webPage = require('webpage');
var page = webPage.create();

// An example url that supports @2x background-image with the following css:-
/* body {
     background-image: background-image: url('http://retinajs.s3.amazonaws.com/images/backgrounds/bg.jpg');
  }
  @media all and (-webkit-min-device-pixel-ratio: 1.5) {
     body {
       background-image: background-image: url('http://retinajs.s3.amazonaws.com/images/backgrounds/bg@2x.jpg');
    }
} */
var examplePage = 'http://imulus.github.io/retinajs/';

//Pass a dpr of your choosing
page.devicePixelRatio = 1.5; // in this case, >=1.5 to get the retina-supporting bg-image

page.open(examplePage, function() {
   var backgroundImage = page.evaluate(function() {
     return window.getComputedStyle(document.body).backgroundImage;
   });
   console.log(backgroundImage); // returns http://retinajs.s3.amazonaws.com/images/backgrounds/bg@2x.jpg
   page.render('2x.png');
});

[Aeon]

+1

[KuroGuo]

+1

[vitallium]

Please send Webkit-specific changes to our submodule repo. Thank you!