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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.shadow.gradient.basic
-// Description:Shadows are drawn for gradient fills
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("Shadows are drawn for gradient fills");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-var gradient = ctx.createLinearGradient(0, 0, 100, 0);
-gradient.addColorStop(0, '#f00');
-gradient.addColorStop(1, '#f00');
-ctx.fillStyle = '#f00';
-ctx.fillRect(0, 0, 100, 50);
-ctx.shadowColor = '#0f0';
-ctx.shadowOffsetY = 50;
-ctx.fillStyle = gradient;
-ctx.fillRect(0, -50, 100, 50);
-_assertPixel(offscreenCanvas, 50,25, 0,255,0,255, "50,25", "0,255,0,255");
-
-t.done();
-
-});
-done();