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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.composite.operation.get
-// 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');
-
-var modes = ['source-atop', 'source-in', 'source-out', 'source-over',
-    'destination-atop', 'destination-in', 'destination-out', 'destination-over',
-    'lighter', 'copy', 'xor'];
-for (var i = 0; i < modes.length; ++i)
-{
-    ctx.globalCompositeOperation = modes[i];
-    _assertSame(ctx.globalCompositeOperation, modes[i], "ctx.globalCompositeOperation", "modes[\""+(i)+"\"]");
-}
-
-t.done();
-
-});
-done();