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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.shadow.alpha.2
-// Description:Shadow colour alpha components are used
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("Shadow colour alpha components are used");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-ctx.fillStyle = '#f00';
-ctx.fillRect(0, 0, 100, 50);
-ctx.shadowColor = 'rgba(0, 0, 255, 0.5)';
-ctx.shadowOffsetY = 50;
-ctx.fillRect(0, -50, 100, 50);
-_assertPixelApprox(offscreenCanvas, 50,25, 127,0,127,255, "50,25", "127,0,127,255", 2);
-
-t.done();
-
-});
-done();