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.compare.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.gradient.object.compare
-// 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 g1 = ctx.createLinearGradient(0, 0, 100, 0);
-var g2 = ctx.createLinearGradient(0, 0, 100, 0);
-_assertDifferent(g1, g2, "g1", "g2");
-ctx.fillStyle = g1;
-_assertSame(ctx.fillStyle, g1, "ctx.fillStyle", "g1");
-
-t.done();
-
-});
-done();