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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.composite.operation.casesensitive
-// 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.globalCompositeOperation = 'xor';
-ctx.globalCompositeOperation = 'Source-over';
-_assertSame(ctx.globalCompositeOperation, 'xor', "ctx.globalCompositeOperation", "'xor'");
-
-t.done();
-
-});
-done();