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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.composite.transparent.copy
-// 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 = 'rgba(0, 255, 0, 0.5)';
-ctx.fillRect(0, 0, 100, 50);
-ctx.globalCompositeOperation = 'copy';
-ctx.fillStyle = 'rgba(0, 0, 255, 0.75)';
-ctx.fillRect(0, 0, 100, 50);
-_assertPixelApprox(offscreenCanvas, 50,25, 0,0,255,191, "50,25", "0,0,255,191", 5);
-
-t.done();
-
-});
-done();