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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/compositing/2d.composite.globalAlpha.imagepattern.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.composite.globalAlpha.imagepattern
-// Description:
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-ctx.fillStyle = '#0f0';
-ctx.fillRect(0, 0, 100, 50);
-var promise = new Promise(function(resolve, reject) {
-    var xhr = new XMLHttpRequest();
-    xhr.open("GET", '/images/red.png');
-    xhr.responseType = 'blob';
-    xhr.send();
-    xhr.onload = function() {
-        resolve(xhr.response);
-    };
-});
-promise.then(function(response) {
-    ctx.fillStyle = ctx.createPattern(response, 'no-repeat');
-    ctx.globalAlpha = 0.01; // avoid any potential alpha=0 optimisations
-    ctx.fillRect(0, 0, 100, 50);
-    _assertPixelApprox(offscreenCanvas, 50,25, 2,253,0,255, "50,25", "2,253,0,255", 2);
-});
-
-t.done();
-
-});
-done();