HTML <canvas>: add WEBP lossless encoding support

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

+<canvas style="display: none"></canvas>
+<!-- The <img> background color should be visible behind the html5.png test image: the
+  image has an alpha channel, which should be preserved by the WEBP toDataURL encoder. -->
+<img id="result" style="background-color: yellow">
+<pre id="error"></pre>
+
+<script>
+if (window.testRunner) {
+    window.testRunner.dumpAsTextWithPixelResults();
+    window.testRunner.waitUntilDone();
+}
+
+var image = new Image();
+
+image.onload = function() {
+    var canvas = document.querySelector('canvas');
+    canvas.width = this.width * 2;
+    canvas.height = this.height * 2;
+    canvas.getContext('2d').drawImage(this, this.width / 2, this.height / 2);
+
+    var dataURL = canvas.toDataURL('image/webp', 0.8); // good quality
+
+    if (!dataURL.match(/^data:image\/webp[;,]/))
+        error.textContent += "FAIL: the dataURL should have 'image/webp' type.";
+    else
+        result.src = dataURL;
+
+    if (window.testRunner)
+        window.testRunner.notifyDone();
+};
+
+image.src = 'resources/html5.png';
+</script>