Use srcset's resource pixel density to determine intrinsic size

LayoutTests/fast/hidpi/resources/svg_canvas_helper.js

+function printSizes(imageId, outputId) {
+  var image = document.getElementById(imageId);
+  image.addEventListener('load', function() {
+    var output = document.getElementById(outputId);
+    output.innerHTML = 'image stats: '
+        + 'size(' + image.width + ',' + image.height + '), '
+        + 'naturalSize(' + image.naturalWidth + ',' + image.naturalHeight + ')<br/>'
+        + 'image codes: '
+        + image.outerHTML.replace(/</g, "&lt;").replace(/>/g, "&gt;");
+  });
+}
+
+function drawCanvas(imageId, canvasId, canvasWidth, canvasHeight) {
+  var image = document.getElementById(imageId);
+  var canvas = document.getElementById(canvasId);
+  canvas.width = canvasWidth;
+  canvas.height = canvasHeight;
+  var context = canvas.getContext('2d');
+  context.drawImage(image, 0, 0);
+}