HTML <canvas>: add WEBP lossless encoding support

third_party/WebKit/LayoutTests/fast/canvas/canvas-toDataURL-webp-maximum-quality.html

+<!-- The letters in the right image should be crisp like the letters in the left image. -->
+<canvas></canvas>
+<img id="result">
+<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;
+    canvas.height = this.height;
+    canvas.getContext('2d').drawImage(this, 0, 0);
+
+    var dataURL = canvas.toDataURL('image/webp', 1.0); // maximum 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/letters.png";
+</script>