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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/shadows/2d.shadow.image.transparent.1.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.shadow.image.transparent.1
-// Description:Shadows are not drawn for transparent images
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("Shadows are not drawn for transparent images");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-ctx.fillStyle = '#0f0';
-ctx.fillRect(0, 0, 100, 50);
-ctx.shadowColor = '#f00';
-ctx.shadowOffsetY = 50;
-var promise = new Promise(function(resolve, reject) {
-    var xhr = new XMLHttpRequest();
-    xhr.open("GET", '/images/transparent.png');
-    xhr.responseType = 'blob';
-    xhr.send();
-    xhr.onload = function() {
-        resolve(xhr.response);
-    };
-});
-promise.then(function(response) {
-    ctx.drawImage(response, 0, -50);
-    _assertPixel(offscreenCanvas, 50,25, 0,255,0,255, "50,25", "0,255,0,255");
-});
-
-t.done();
-
-});
-done();