Implement convertToBlob() in OffscreenCanvas

third_party/WebKit/LayoutTests/virtual/threaded/fast/idleToBlob/OffscreenCanvas-convertToBlob-2d-main-expected.html

+<img id="png"/>
+<img id="jpeg-high"/>
+<img id="jpeg-low"/>
+<img id="webp-high"/>
+<img id="webp-low"/>
+<script>
+if (window.testRunner) {
+  testRunner.waitUntilDone();
+}
+
+var pngImage = document.getElementById('png');
+var jpegImageHigh = document.getElementById('jpeg-high');
+var jpegImageLow = document.getElementById('jpeg-low');
+var webpImageHigh = document.getElementById('webp-high');
+var webpImageLow = document.getElementById('webp-low');
+var numTestCount = 5;
+function imageLoaded() {
+  numTestCount--;
+  if (numTestCount == 0 && window.testRunner) {
+    window.testRunner.notifyDone();
+  }
+}
+pngImage.addEventListener('load', imageLoaded);
+jpegImageHigh.addEventListener('load', imageLoaded);
+jpegImageLow.addEventListener('load', imageLoaded);
+webpImageHigh.addEventListener('load', imageLoaded);
+webpImageLow.addEventListener('load', imageLoaded);
+
+var canvas = document.createElement("canvas");
+canvas.width = 50;
+canvas.height = 50;
+var ctx = canvas.getContext('2d');
+ctx.fillStyle = "red";
+ctx.fillRect(0, 0, 25, 25);
+ctx.fillStyle = "green";
+ctx.fillRect(25, 0, 25, 25);
+ctx.fillStyle = "blue";
+ctx.fillRect(0, 25, 25, 25);
+ctx.fillStyle = "black";
+ctx.fillRect(25, 25, 25, 25);
+ctx.strokeStyle = "yellow";
+ctx.strokeRect(0, 0, 50, 50);
+
+canvas.toBlob(function(blob) {
+  pngImage.src = URL.createObjectURL(blob);
+});
+
+canvas.toBlob(function(blob) {
+  jpegImageHigh.src = URL.createObjectURL(blob);
+}, "image/jpeg", 1.0);
+
+canvas.toBlob(function(blob) {
+    jpegImageLow.src = URL.createObjectURL(blob);
+}, "image/jpeg", 0.2)
+
+canvas.toBlob(function(blob) {
+    webpImageHigh.src = URL.createObjectURL(blob);
+}, "image/webp", 1.0);
+
+canvas.toBlob(function(blob) {
+    webpImageLow.src = URL.createObjectURL(blob);
+}, "image/webp", 0.2);
+
+</script>
+