Update pixel test expectation and add new worker tests

third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-commit-worker.html

-<!DOCTYPE>
-<canvas id="canvas2D" width="50" height="50" background-color="red"></canvas>
-<script id="myWorker" type="text/worker">
-self.onmessage = function(e) {
-    var transferredOffscreenCanvas = e.data.data;
-    var offscreen2d = transferredOffscreenCanvas.getContext("2d");
-    offscreen2d.commit();
-    self.postMessage("");
-};
-</script>
-<script>
-// Note: If someone see this test flaky in the future, please mark the test as
-// flaky; the code itself is not the cause of such flakiness.
-// TODO(xlai): Guarantee all OffscreenCanvas commit tests non-flaky under all cases.
-// See crbug.com/645173.
-if (window.testRunner) {
-    testRunner.dumpAsTextWithPixelResults();
-    testRunner.waitUntilDone();
-}
-
-function makeWorker(script) {
-   var blob = new Blob([script]);
-   return new Worker(URL.createObjectURL(blob));
-}
-
-var canvas2D = document.getElementById("canvas2D");
-var offscreenCanvas = canvas2D.transferControlToOffscreen();
-var worker = makeWorker(document.getElementById("myWorker").textContent);
-worker.onmessage = function (e) {
-    if (window.testRunner) testRunner.notifyDone();
-};
-worker.postMessage({data: offscreenCanvas}, [offscreenCanvas]);
-
-</script>