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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/path-objects/2d.path.arcTo.negative.worker.js

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