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

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

-// DO NOT EDIT! This test has been generated by tools/gentest.py.
-// OffscreenCanvas test in a worker:2d.line.defaults
-// 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');
-
-_assertSame(ctx.lineWidth, 1, "ctx.lineWidth", "1");
-_assertSame(ctx.lineCap, 'butt', "ctx.lineCap", "'butt'");
-_assertSame(ctx.lineJoin, 'miter', "ctx.lineJoin", "'miter'");
-_assertSame(ctx.miterLimit, 10, "ctx.miterLimit", "10");
-
-t.done();
-
-});
-done();