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

third_party/WebKit/LayoutTests/imported/wpt/offscreen-canvas/line-styles/2d.line.join.valid.worker.js

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.line.join.valid
-// Description:Setting lineJoin to valid values works
-// Note:
-
-importScripts("/resources/testharness.js");
-importScripts("/common/canvas-tests.js");
-
-var t = async_test("Setting lineJoin to valid values works");
-t.step(function() {
-
-var offscreenCanvas = new OffscreenCanvas(100, 50);
-var ctx = offscreenCanvas.getContext('2d');
-
-ctx.lineJoin = 'bevel'
-_assertSame(ctx.lineJoin, 'bevel', "ctx.lineJoin", "'bevel'");
-ctx.lineJoin = 'round';
-_assertSame(ctx.lineJoin, 'round', "ctx.lineJoin", "'round'");
-ctx.lineJoin = 'miter';
-_assertSame(ctx.lineJoin, 'miter', "ctx.lineJoin", "'miter'");
-
-t.done();
-
-});
-done();