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.radial.negative.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.gradient.radial.negative
-// Description:createRadialGradient() throws INDEX_SIZE_ERR if either radius is negative
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("createRadialGradient() throws INDEX_SIZE_ERR if either radius is negative");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-assert_throws("INDEX_SIZE_ERR", function() { ctx.createRadialGradient(0, 0, -0.1, 0, 0, 1); });
-assert_throws("INDEX_SIZE_ERR", function() { ctx.createRadialGradient(0, 0, 1, 0, 0, -0.1); });
-assert_throws("INDEX_SIZE_ERR", function() { ctx.createRadialGradient(0, 0, -0.1, 0, 0, -0.1); });
-
-t.done();
-
-});
-done();