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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/fill-and-stroke-styles/2d.gradient.object.update.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.gradient.object.update
-// 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 g = ctx.createLinearGradient(-100, 0, 200, 0);
-g.addColorStop(0, '#f00');
-g.addColorStop(1, '#f00');
-ctx.fillStyle = g;
-g.addColorStop(0.1, '#0f0');
-g.addColorStop(0.9, '#0f0');
-ctx.fillRect(0, 0, 100, 50);
-_assertPixelApprox(offscreenCanvas, 50,25, 0,255,0,255, "50,25", "0,255,0,255", 2);
-
-t.done();
-
-});
-done();