Enforce OffscreenCanvas.convertToBlob to terminate after idling for too long

third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-convertToBlob-noIdleTask.html

+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<script type = 'text/javascript'>
+// In blink Layout Tests, flag "--enable-threaded-compositing", which is
+// essential for idle task running, is not turned on by default. This makes
+// it convenient to test the case when CanvasAsyncBlobCreator is idling for
+// too long and switching to forcing encoding in a normal task.
+// Passing this test means that the enforcing mechanism in
+// CanvasAsyncBlobCreator for OffscreenCanvas case is working as expected.
+var w = 4;
+var h = 4;
+
+var newImg = new Image();
+function imageLoaded() {
+    var canvas2 = document.createElement("canvas");
+    canvas2.width = w;
+    canvas2.height = h;
+    var ctx2 = canvas2.getContext("2d");
+    ctx2.drawImage(newImg, 0, 0, w, h);
+
+    var imageData = ctx2.getImageData(0, 0, w, h).data;
+    assert_equals(imageData[0], 255);
+    assert_equals(imageData[1], 1);
+    assert_equals(imageData[2], 2);
+    assert_equals(imageData[3], 255);
+    testImageFromOffscreen.done();
+}
+
+var testImageFromOffscreen = async_test(
+    "Check if the image loaded from blob returned by " +
+    "OffscreenCanvas.convertToBlob() have expected image data values.");
+
+testImageFromOffscreen.step(function() {
+  newImg.onload = testImageFromOffscreen.step_func(imageLoaded);
+
+  var offscreenCanvas = new OffscreenCanvas(w, h);
+  var ctx = offscreenCanvas.getContext("2d");
+  ctx.fillStyle = "#FF0102";
+  ctx.fillRect(0, 0, w, h);
+
+  offscreenCanvas.convertToBlob().then(function(blob) {
+      newImg.src = URL.createObjectURL(blob);
+  });
+});
+
+</script>