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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/shadows/2d.shadow.pattern.alpha.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.shadow.pattern.alpha
-// Description:Shadows are drawn correctly for partially-transparent fill patterns
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("Shadows are drawn correctly for partially-transparent fill patterns");
-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/transparent50.png');
-    xhr.responseType = 'blob';
-    xhr.send();
-    xhr.onload = function() {
-        resolve(xhr.response);
-    };
-});
-promise.then(function(response) {
-    var pattern = ctx.createPattern(response, 'repeat');
-    ctx.fillStyle = '#f00';
-    ctx.fillRect(0, 0, 100, 50);
-    ctx.shadowOffsetY = 50;
-    ctx.shadowColor = '#00f';
-    ctx.fillStyle = pattern;
-    ctx.fillRect(0, -50, 100, 50);
-});
-
-t.done();
-
-});
-done();