Support lossy and lossless <canvas>.toDataURL for webp

LayoutTests/fast/canvas/canvas-toBlob-webp-lossless.html

+<!-- The letters in the right image should be crisp like the letters in the left image. -->
+<canvas></canvas>
+<img id="result" onload="testDone()">
+
+<script>
+if (window.testRunner) {
+    testRunner.dumpAsTextWithPixelResults();
+    testRunner.waitUntilDone();
+}
+
+function testDone()
+{
+    if (window.testRunner)
+        testRunner.notifyDone();
+}
+
+var image = new Image();
+image.onload = function() {
+    var canvas = document.querySelector('canvas');
+    canvas.width = image.width;
+    canvas.height = image.height;
+    canvas.getContext('2d').drawImage(image, 0, 0);
+
+    document.body.style.zoom = 1.3;
+
+    canvas.toBlob(function(blob) {
+        var failure = "../../fast/images/resources/rgb-jpeg-red.jpg";
+        if (!blob) {
+            result.src = failure;
+        } else if (blob.type != 'image/webp') {
+            result.src = failure;
+        } else {
+            result.src = URL.createObjectURL(blob);
+        }
+    }, "image/webp.ll", 0.2);
+};
+
+image.src = "resources/letters.png";
+</script>