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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.shadow.offset.positiveY
-// Description:Shadows can be offset with positive y
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("Shadows can be offset with positive y");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-ctx.fillStyle = '#f00';
-ctx.fillRect(0, 0, 100, 50);
-ctx.fillStyle = '#0f0';
-ctx.shadowColor = '#0f0';
-ctx.shadowOffsetY = 25;
-ctx.fillRect(0, 0, 100, 25);
-_assertPixel(offscreenCanvas, 50,12, 0,255,0,255, "50,12", "0,255,0,255");
-_assertPixel(offscreenCanvas, 50,37, 0,255,0,255, "50,37", "0,255,0,255");
-
-t.done();
-
-});
-done();