Add offscreen-canvas to the list of wpt directories to skip.

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/drawing-images-to-the-canvas/2d.drawImage.animated.poster.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.drawImage.animated.poster
-// Description:drawImage() of an APNG draws the poster frame
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("drawImage() of an APNG draws the poster frame");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-var promise = new Promise(function(resolve, reject) {
-    var xhr = new XMLHttpRequest();
-    xhr.open("GET", '/images/anim-poster-gr.png');
-    xhr.responseType = 'blob';
-    xhr.send();
-    xhr.onload = function() {
-        resolve(xhr.response);
-    };
-});
-promise.then(function(response) {
-    ctx.drawImage(response, 0, 0);
-    _assertPixelApprox(offscreenCanvas, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 2);
-});
-
-t.done();
-
-});
-done();