Support lossy and lossless <canvas>.toDataURL for webp

LayoutTests/fast/canvas/canvas-toDataURL-webp-alpha.html

 <canvas id="canvas" width="64px" height="64px" style="display: none"></canvas>
+<!-- The <img> background color should be visible behind the html5.png test image (the test
+  image has an alpha channel, which should be preserved by the webp toDataURL encoder). -->
+<img id="result" style="background-color: green">
 <pre id="log"></pre>
-<img id="result">
+
 <script>
 if (window.testRunner) {
     window.testRunner.dumpAsTextWithPixelResults();
     window.testRunner.waitUntilDone();
 }
 
-function webpDataURLTest()
-{
+var image = new Image();
+image.onload = function() {
     var canvas = document.getElementById('canvas');
-    var context = canvas.getContext('2d');
-    context.drawImage(window.image, 0, 0, canvas.width, canvas.height);
+    canvas.getContext('2d').drawImage(this, 0, 0, canvas.width, canvas.height);
 
     var dataURL = canvas.toDataURL('image/webp', 0.8);
+
     if (!dataURL.match(/^data:image\/webp[;,]/))
-        document.getElementById('log').textContent += "FAIL: canvas.toDataURL('image/webp') not supported";
+        document.getElementById('log').textContent += "FAIL: the dataURL should have 'image/webp' type.";
     else
         document.getElementById('result').src = dataURL;
 
     if (window.testRunner)
         window.testRunner.notifyDone();
-}
+};
 
-var image = new Image();
-image.onload = webpDataURLTest;
 image.src = 'resources/html5.png';
 </script>